Added Preamble (#2954)

... to let the file point to its canonical source

Unity.gitignore

@@ -1,5 +1,7 @@
 # This .gitignore file should be placed at the root of your Unity project directory
-
+#
+# Get latest from https://github.com/github/gitignore/blob/master/Unity.gitignore
+#
 /[Ll]ibrary/
 /[Tt]emp/
 /[Oo]bj/