Add instructions to run specific tests (#660)

Runing the full tests using make test is very slow. For most PRs, we may work on only a small
fraction of the codes, and it's common that we only need to run specific tests.

This PR adds instructions for running specific tests.

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

Author: seisman

CONTRIBUTING.md

@@ -308,6 +308,14 @@ in your browser. **Strive to get 100% coverage for the lines you changed.**
 It's OK if you can't or don't know how to test something.
 Leave a comment in the PR and we'll help you out.
 
+You can also run tests in just one test script using:
+
+    pytest --verbose --mpl --mpl-results-path=results --doctest_modules pygmt/tests/NAME_OF_TEST_FILE.py
+
+or run tests which contain names that match a specific keyword expression:
+
+    pytest --verbose --mpl --mpl-results-path=results --doctest_modules -k KEYWORD pygmt/tests
+
 ### Testing plots
 
 Writing an image-based test is only slightly more difficult than a simple test.