Update CONTRIBUTING.md

Summary: Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: strulovich

Differential Revision: D34512702

fbshipit-source-id: f400ba11d0f83e254e93eb96fb56d4ff9d304b74
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 745cb05..75522ca 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -2,6 +2,8 @@
 We want to make contributing to this project as easy and transparent as
 possible.
 
+**If you plan a significant change, please first start a discussion as a GitHub Issue.**
+
 ## Pull Requests
 We actively welcome your pull requests.