Add AUTHORS list crediting developers

(cherry picked from commit 21bcb71b2c)

AUTHORS.md

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+# Godot Engine authors
+
+Godot Engine is developed by a community of voluntary contributors who
+contribute code, bug reports, documentation, artwork, support, etc.
+
+It is impossible to list them all; nevertheless, this file aims at listing
+the developers who contributed significant patches to this MIT licensed
+source code. "Significant" is arbitrarily decided, but should be fair :)
+
+GitHub usernames are indicated in parentheses, or as sole entry when no other
+name is available.
+
+## Original authors
+
+    Juan Linietsky (reduz)
+    Ariel Manzur (punto-)
+
+## Developers
+
+(in alphabetical order, with 10 commits or more excluding merges)
+
+    Alexander Holland (AlexHolly)
+    Alexey Velikiy (jonyrock)
+    Andreas Haas (Hinsbart)
+    Anton Yabchinskiy (a12n)
+    Aren Villanueva (kurikaesu)
+    Ariel Manzur (punto-)
+    Bastiaan Olij (BastiaanOlij)
+    Bojidar Marinov (bojidar-bg)
+    Błażej Szczygieł (zaps166)
+    Carl Olsson (not-surt)
+    Dana Olson (adolson)
+    Daniel J. Ramirez (djrm)
+    Fabio Alessandrelli (Faless)
+    Ferenc Arn (tagcup)
+    Franklin Sobrinho (TheHX)
+    Geequlim
+    Gen (dbsGen)
+    George Marques (vnen)
+    Guilherme Felipe (guilhermefelipecgs)
+    Hein-Pieter van Braam (hpvb)
+    Hubert Jarosz (Marqin)
+    Ignacio Etcheverry (neikeq)
+    J08nY
+    Johan Manuel (29jm)
+    Joshua Grams (JoshuaGrams)
+    Juan Linietsky (reduz)
+    Julian Murgia (StraToN)
+    Kostadin Damyanov (Max-Might)
+    L. Krause (eska014)
+    Marcelo Fernandez (marcelofg55)
+    Mariano Javier Suligoy (MarianoGnu)
+    Mario Schlack (hurikhan)
+    Masoud BH (masoudbh3)
+    Nathan Warden (NathanWarden)
+    Ovnuniarchos
+    Patrick (firefly2442)
+    Paul Batty (Paulb23)
+    Pawel Kowal (pkowal1982)
+    Pedro J. Estébanez (RandomShaper)
+    Ralf Hölzemer (rollenrolm)
+    RayKoopa
+    Rémi Verschelde (akien-mga)
+    SaracenOne
+    Thomas Herzog (karroffel)
+    V. Vamsi Krishna (vkbsb)
+    Vinzenz Feenstra (vinzenz)
+    Zher Huei Lee (leezh)
+    ZuBsPaCe
+    박한얼 (volzhs)
+    est31
+    marynate
+    mrezai
+    romulox-x
+    sanikoyes
+    yg2f (SuperUserNameMan)