Update RGBDS history to mention org move

src/rgbds.7

@@ -45,4 +45,8 @@ It is now maintained by a number of contributors at
 .Lk https://github.com/rednex/rgbds .
 .It
 2018, codebase relicensed under the MIT license.
+.It
+2020, repository is moved to the gbdev organisation. The
+.Lk https://rgbds.gbdev.io
+website serving documentation and downloads is created.
 .El