Update docs on how to contribute translations

Now translation contributions are made in the Pontoon platform. This
change makes the docs reflect this.

Bug: twpowertools:40
Change-Id: Ie9adec57bc03d2913842a69d47deb097bff40cb3
diff --git a/docs/contributing.md b/docs/contributing.md
index 53fe1b0..0cdd4f2 100644
--- a/docs/contributing.md
+++ b/docs/contributing.md
@@ -21,10 +21,10 @@
 ## Contribute translations
 If you speak a language other than English, you can help translate the UI to
-your language!
+your language! Just go to the following website and select your language to
+start translating:
-If you're interested in helping with this, please email me at
-`me [at] avm99963.com`.
+- https://avm99963-pontoon.herokuapp.com/projects/tw-power-tools/
 ## Beta test the extension
 Releasing an update to the stable channel isn't always an easy task, because