[DOC] refine the contributing documentation based on the latest updates

[skip travis]

CONTRIBUTING.md

@@ -5,8 +5,9 @@ just a few small guidelines you need to follow.
 
 ## Submission and Testing Process
 
-- Submit a Pull Request to the [develop](https://github.com/efabless/openlane/tree/develop) branch only. <br> Check [Code Reviews](#code_reviews) for more details.
+- Submit a Pull Request to the [master](https://github.com/efabless/openlane/tree/master) branch only. <br> Check [Code Reviews](#code_reviews) for more details.
 - Our CI that would test your PR once submitted, yet it would be nice for you to run a couple of tests from your end to shorten the cycle of reviews. For that purpose, you can use:
+    - `make fastest_test_set`: to run the same test set that the basic CI uses, which will be used to evaluate your Pull Request.
     - [This](./regression_results/README.md) for custom test sets. (check the `-b` flag).
     - `make test`: tests the flow against one design `$TEST_DESIGN`. The default is `spm`.
     - `make regression_test`: tests the flow against all available designs and compares the resulting statistics with benchmark results and produces a human readable report and summary.