Suggest forking.

CONTRIBUTING.md

@@ -1,5 +1,7 @@
 This guide is primarily intended to be _descriptive_, detailing the usage
 that Swift developers at GitHub have arrived at in cases of disagreement.
+Consider whether a proposed addition would make more sense in a fork used
+by your projects/organization instead.
 
 If you want to suggest a change or addition that will help accomplish
 [the goals](README.md) of this style guide, please open a pull request that: