[DOC] Update contribution guide link

CONTRIBUTING.md

@@ -0,0 +1,8 @@
+Welcome, Contributor!
+=====================
+
+It's great to see you here - and we welcome your contributions. But first, take a look at
+our official contribution guide which explains what we expect from you - in order to make
+sure you can expect the best possible result from our extensions. Without further ado:
+
+> https://fluidtypo3.org/documentation/contributing/contribution-guide.html