add a few mentions to the RFC process in the repo (#1756)

.github/ISSUE_TEMPLATE/config.yml

@@ -1 +1,5 @@
 blank_issues_enabled: true
+contact_links:
+    - name: 📝 Request For Comments (RFC)
+      url: https://github.com/microsoft/rnx-kit/tree/rfcs
+      about: For proposals that can introduce "substantial" changes to rnx-kit, please follow our RFC process.

README.md

@@ -36,3 +36,6 @@ suggestions!
 Take a look at
 [CONTRIBUTING](https://github.com/microsoft/rnx-kit/tree/main/CONTRIBUTING.md)
 for details.
+
+If you are interested in proposing "substantial" changes, please refer to our
+[RFC process](https://github.com/microsoft/rnx-kit/tree/rfcs).