Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -1,3 +1,17 @@
+# Signing the CLA
+
+Before contributing to this project, you must sign [the CLA](/cla/cla-1.0.0.md).
+
+To sign the CLA, add your GitHub username to the end of the CLA. Make sure that the usernames remain alphabetically sorted.
+
+Then create a pull request with the title:
+
+> Sign CLA
+
+and with the body:
+
+> I have read the Agreement, and fully agree to it by signing it with my GitHub username.
+
 # Coding standards
 
 ## Literal structure