CONTRIBUTING: add explicit link to ECA

This was requested in the 5.12 release review.

Bug: https://gitlab.eclipse.org/eclipsefdn/emo-team/emo/-/issues/58
Change-Id: I21abe960412a7bc7a0d2f3bdfc4a1d031a700c3b

CONTRIBUTING.md

@@ -1,6 +1,7 @@
 # Contributing
 
 Please refer to the [contributor guide](https://wiki.eclipse.org/EGit/Contributor_Guide) for all the details.
+Contributions require that you sign the [Eclipse Contributor Agreement](https://www.eclipse.org/legal/ECA.php).
 
 ## Reporting bugs