Added inclusion phrase for meta files

gitignore template of visualstudio is excluding meta file

a proposal like in this issue could make use of the inclusion phrases to make generated gitignore files more usable: https://github.com/toptal/gitignore.io/issues/626

Unity.gitignore

@@ -47,7 +47,8 @@ ExportedObj/
 *.opendb
 *.VC.db
 
-# Unity3D generated meta files
+# Unity3D generated meta files (include & exclude)
+!*.meta
 *.pidb.meta
 *.pdb.meta
 *.mdb.meta