Update CONTRIBUTING.md
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d2c2c50..f1fcd4d 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -20,7 +20,7 @@
To get the code:
- * Fork `https://github.com/flutter/engine` into your own GitHub account.
+ * Fork `https://github.com/flutter/engine` into your own GitHub account. If you already have a fork, and are now installing a development environment on a new machine, make sure you've updated your fork so that you don't use stale configuration options from long ago.
* If you haven't configured your machine with an SSH key that's shared by github then
follow the directions here: https://help.github.com/articles/generating-ssh-keys/.
* Create an empty directory for your copy of the repository. Call it what you like. For
