Add note re only adding tests with Free Software fonts

test/shaping/README.md

@@ -30,8 +30,11 @@ what this does is:
 $ ./hb-unicode-encode 41 42 43 627 | ./record-it.sh ../../util/hb-shape font.ttf >> tests/test-name.test
 ```
 
-If you created a new test file, add it to Makefile.am so it is run.
+If you created a new test file, add it to `Makefile.am` so it is run.
 Check that `make test` does indeed run it, and that the test passes.
 When everything looks good, `git add` the new font as well as new
 test file if you created any.  You can see what new files are there
 by running `git status tests fonts/sha1sum`.  And commit!
+
+*Note!*  Please only add tests using Open Source fonts, preferably under
+OFL or similar license.