Added citation to MacOS.gitignore and Windows.gitignore

I wanted to add these files to my repositories. but felt responsible to include a citation to the source.

Global/Windows.gitignore

@@ -1,3 +1,5 @@
+## Get latest from https://github.com/github/gitignore/
+
 # Windows thumbnail cache files
 Thumbs.db
 Thumbs.db:encryptable

Global/macOS.gitignore

@@ -1,3 +1,5 @@
+## Get latest from https://github.com/github/gitignore/
+
 # General
 .DS_Store
 .AppleDouble