Improve platform-specific READMEs to add useful links

This also adds READMEs for all platforms.

platform/android/README.md

@@ -3,6 +3,13 @@
 This folder contains the Java and C++ (JNI) code for the Android platform port,
 using [Gradle](https://gradle.org/) as a build system.
 
+## Documentation
+
+- [Compiling for Android](https://docs.godotengine.org/en/latest/development/compiling/compiling_for_android.html)
+  - Instructions on building this platform port from source.
+- [Exporting for Android](https://docs.godotengine.org/en/latest/tutorials/export/exporting_for_android.html)
+  - Instructions on using the compiled export templates to export a project.
+
 ## Artwork license
 
 [`logo.png`](logo.png) and [`run_icon.png`](run_icon.png) are licensed under

platform/ios/README.md

@@ -0,0 +1,14 @@
+# iOS platform port
+
+This folder contains the C++, Objective-C and Objective-C++ code for the iOS
+platform port.
+
+See also [`misc/dist/ios_xcode`](/misc/dist/ios_xcode) folder for the Xcode
+project template used for packaging the iOS export templates.
+
+## Documentation
+
+- [Compiling for iOS](https://docs.godotengine.org/en/latest/development/compiling/compiling_for_ios.html)
+  - Instructions on building this platform port from source.
+- [Exporting for iOS](https://docs.godotengine.org/en/latest/tutorials/export/exporting_for_ios.html)
+  - Instructions on using the compiled export templates to export a project.

platform/javascript/README.md

@@ -5,8 +5,15 @@ compiled using [Emscripten](https://emscripten.org/).
 
 It also contains a ESLint linting setup (see [`package.json`](package.json)).
 
-See also [`misc/dist/html`](/misc/dist/html) folder for files used by this platform
-such as the HTML5 shell.
+See also [`misc/dist/html`](/misc/dist/html) folder for additional files used by
+this platform such as the HTML5 shell.
+
+## Documentation
+
+- [Compiling for the Web](https://docs.godotengine.org/en/latest/development/compiling/compiling_for_web.html)
+  - Instructions on building this platform port from source.
+- [Exporting for the Web](https://docs.godotengine.org/en/latest/tutorials/export/exporting_for_web.html)
+  - Instructions on using the compiled export templates to export a project.
 
 ## Artwork license
 

platform/linuxbsd/README.md

@@ -2,10 +2,20 @@
 
 This folder contains the C++ code for the Linux/*BSD platform port.
 
+See also [`misc/dist/linux`](/misc/dist/linux) folder for additional files
+used by this platform.
+
+## Documentation
+
+- [Compiling for Linux/*BSD](https://docs.godotengine.org/en/latest/development/compiling/compiling_for_linuxbsd.html)
+  - Instructions on building this platform port from source.
+- [Exporting for Linux/*BSD](https://docs.godotengine.org/en/latest/tutorials/export/exporting_for_linux.html)
+  - Instructions on using the compiled export templates to export a project.
+
 ## Artwork license
 
 [`logo.png`](logo.png) is derived from the [Linux logo](https://isc.tamu.edu/~lewing/linux/):
 
 > Permission to use and/or modify this image is granted provided you acknowledge me
-  <lewing@isc.tamu.edu> and [The GIMP](https://isc.tamu.edu/~lewing/gimp/)
-  if someone asks.
+> <lewing@isc.tamu.edu> and [The GIMP](https://isc.tamu.edu/~lewing/gimp/)
+> if someone asks.

platform/macos/README.md

@@ -0,0 +1,19 @@
+# macOS platform port
+
+This folder contains the C++, Objective-C and Objective-C++ code for the macOS
+platform port.
+
+See also [`misc/dist/macos`](/misc/dist/macos) folder for additional files used
+by this platform. [`misc/dist/macos_tools.app`](/misc/dist/macos_tools.app) is
+an `.app` bundle template used for packaging the macOS editor, while
+[`misc/dist/macos_template.app`](/misc/dist/macos_template.app) is used for
+packaging macOS export templates.
+
+## Documentation
+
+- [Compiling for macOS](https://docs.godotengine.org/en/latest/development/compiling/compiling_for_macos.html)
+  - Instructions on building this platform port from source.
+- [Exporting for macOS](https://docs.godotengine.org/en/latest/tutorials/export/exporting_for_macos.html)
+  - Instructions on using the compiled export templates to export a project.
+- [Running Godot apps on macOS](https://docs.godotengine.org/en/latest/tutorials/export/running_on_macos.html)
+  - Instructions on running Godot projects on macOS.

platform/uwp/README.md

@@ -0,0 +1,20 @@
+# UWP platform port
+
+> **Warning**
+>
+> The UWP platform port is not currently in a working state for the `master`
+> branch, and may be dropped in the future.
+
+This folder contains the C++ code for the Universal Windows Platform (UWP)
+platform port. **This is not to be confused with the "standard" Win32 port**,
+which is available in [`platform/windows`](/platform/windows).
+
+See also [`misc/dist/uwp_template`](/misc/dist/uwp_template) folder for the UWP
+project template used for packaging the UWP export templates.
+
+## Documentation
+
+- [Compiling for Universal Windows Platform](https://docs.godotengine.org/en/latest/development/compiling/compiling_for_uwp.html)
+  - Instructions on building this platform port from source.
+- [Exporting for Universal Windows Platform](https://docs.godotengine.org/en/latest/tutorials/export/exporting_for_uwp.html)
+  - Instructions on using the compiled export templates to export a project.

platform/windows/README.md

@@ -0,0 +1,15 @@
+# Windows platform port
+
+This folder contains the C++ and JavaScript code for the Windows platform port.
+
+See also [`misc/dist/windows`](/misc/dist/windows) folder for additional files
+used by this platform.
+
+## Documentation
+
+- [Compiling for Windows](https://docs.godotengine.org/en/latest/development/compiling/compiling_for_windows.html)
+  - Instructions on building this platform port from source.
+- [Exporting for Windows](https://docs.godotengine.org/en/latest/tutorials/export/exporting_for_windows.html)
+  - Instructions on using the compiled export templates to export a project.
+- [Changing application icon for Windows](https://docs.godotengine.org/en/stable/tutorials/export/changing_application_icon_for_windows.html)
+  - Instructions on using a custom icon for the exported project executable.