doc: add README to test-projects directory (#722)

Help users end up in the right place.

Author: david-christiansen

test-projects/README.md

@@ -0,0 +1,9 @@
+# Test Projects for Verso Development
+
+This directory contains test projects that are used during the
+development of Verso. Their build processes integrate with that of
+Verso itself, and using them on a standalone basis requires
+substantial changes.
+
+Sarting points for projects can be found in the
+[Verso templates repository](https://github.com/leanprover/verso-templates).