added SourceSort to how-to-contribute article

Author: robcotter

_articles/how-to-contribute.md

@@ -224,6 +224,7 @@ You can also use one of the following resources to help you discover and contrib
 * [Up For Grabs](https://up-for-grabs.net/)
 * [Contributor-ninja](https://contributor.ninja)
 * [First Contributions](https://firstcontributions.github.io)
+* [SourceSort](https://sourcesort.com/)
 
 ### A checklist before you contribute