Add code owners for specific paths in the codebase

https://blog.github.com/2017-07-06-introducing-code-owners/

CODEOWNERS

@@ -0,0 +1,37 @@
+# Lines starting with '#' are comments.
+# Each line is a file pattern followed by one or more owners.
+# Owners can be @users, @org/teams or emails
+
+core/*            @reduz
+
+doc/*             @godotengine/documentation
+
+drivers/gles2/*   @karroffel
+drivers/gles3/*   @reduz
+
+editor/icons/*    @djrm
+
+main/*            @reduz
+
+misc/*            @akien-mga
+
+modules/bullet/*     @AndreaCatania
+modules/enet/*       @godotengine/network
+modules/gnative/*    @karroffel
+modules/gdscript/*   @reduz @vnen @bojidar-bg
+modules/mbedtls/*    @godotengine/network
+modules/mobile_vr/*  @BastiaanOlij
+modules/mono/*       @neikeq
+modules/regex/*      @LeeZH
+modules/upnp/*       @godotengine/network
+modules/websocket/*  @godotengine/network
+
+platform/javascript/*  @eska014
+platform/uwp/*         @vnen
+
+scene/main/*      @reduz
+
+server/physics*   @reduz @AndreaCatania
+server/visual*    @reduz @karroffel
+
+thirdparty/*      @akien-mga