Added a line to thanks volunteers in credits

I added "And the volunteers who contribute to this project!" at the bottom of the credits in the "about Jami" pop-up.
It's already on the jami-client-android version.

Issue #1689 on gitlab

Change-Id: I59967e5b9e1e7eac0519c9b4960692c14c80dab7

resources/misc/projectcredits.html

@@ -70,3 +70,4 @@ Yang Wang<br></p>
 <h4 align="left"><span style="font-weight:600"> Artwork by</span></h4>
 <p>Charlotte Hoffmann<br>
 Marianne Forget<br></p>
+<h4 align="left"><span style="font-weight:600"> And the volunteers who contribute to this project! </span></h4>