Prepend to TestFlight CHANGELOG

Fixes #598

CHANGELOG.txt.testflight

@@ -0,0 +1,14 @@
+Do not worry, your former profiles ARE NOT LOST.
+
+You are not seeing them just because they were not
+migrated (yet) to version 3.0.0. Downgrading the app
+to the App Store version would restore them.
+
+You may also notice that several features are missing,
+but they will be gradually reintroduced before hitting
+the App Store.
+
+Anything else, please report to beta@passepartoutvpn.app
+
+### Changes
+

fastlane/Fastfile

@@ -46,7 +46,9 @@ end
 
 desc "Push a new beta build to TestFlight"
 lane :beta do
+    prelog = File.read("../#{logname}.testflight")
     log = File.read("../#{logname}")
+    changelog = prelog + "\n\n" + log
     match(type: "appstore")
     gym(
         clean: true,
@@ -54,7 +56,7 @@ lane :beta do
         derived_data_path: "build/derived_data"
     )
     pilot(
-        changelog: log,
+        changelog: changelog,
         distribute_external: true,
         notify_external_testers: false
     )