runtime(doc): Emphasize adding tests when contributing

Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: chrisbra

CONTRIBUTING.md

@@ -12,10 +12,10 @@ A pull request has the advantage that it will trigger the Continuous
 Integration tests, you will be warned of problems (you can ignore the coverage
 warning, it's noisy).
 
-Please consider adding a test. All new functionality should be tested and bug
-fixes should be tested for regressions: the test should fail before the fix and
-pass after the fix. Look through recent patches for examples and find help
-with ":help testing". The tests are located under "src/testdir".
+Please always add a test, if possible. All new functionality should be tested
+and bug fixes should be tested for regressions: the test should fail before the
+fix and pass after the fix. Look through recent patches for examples and find
+help with ":help testing". The tests are located under "src/testdir".
 
 Contributions will be distributed with Vim under the Vim license. Providing a
 change to be included implies that you agree with this and your contribution