Add AUTHORS list crediting developers

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)