Mention RFC process in contribution instructions (#14291)

.github/ISSUE_TEMPLATE/feature-request.md

@@ -10,3 +10,7 @@ labels: kind:feature
 - Describe the feature you would like, optionally illustrated by examples, and how it will solve the above problem.
 - Describe considered alternative solutions, and the reasons why you have not proposed them as a solution here.
 - Does it break backward compatibility, if yes then what's the migration path?
+
+In case this proposal includes a substantial change to the language, we ask you to go through an [RFC process](https://github.com/crystal-lang/rfcs).
+
+The best place to start an open discussion about potential changes is the [Crystal forum](https://forum.crystal-lang.org/c/crystal-contrib/6).

CONTRIBUTING.md

@@ -15,6 +15,10 @@ there's no more room for discussion. We'll anyway close the issue after some day
 
 If something is missing from the language it might be that it's not yet implemented or that it was purposely left out. If in doubt, just ask.
 
+Substantial changes go through an [RFC process](https://github.com/crystal-lang/rfcs).
+
+The best place to start an open discussion about potential changes is the [Crystal forum](https://forum.crystal-lang.org/c/crystal-contrib/6).
+
 ### What's needed right now
 
 You can find a list of tasks that we consider suitable for a first time contribution at