CONTRIBUTING: add excplicit link to ECA

This was requested in the 5.12 release review.

Bug: https://gitlab.eclipse.org/eclipsefdn/emo-team/emo/-/issues/59
Change-Id: I853c7a2e14ee52a33ef7d829971aba4780768923

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 439a523..5738fc4 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,6 +1,7 @@
 # Contributing
 
 Please refer to the [contributor guide](https://wiki.eclipse.org/EGit/GitHub/Contributor_Guide) for all the details.
+Contributions require that you sign the [Eclipse Contributor Agreement](https://www.eclipse.org/legal/ECA.php).
 
 ## Reporting bugs