updates to CONTRIBUTING

Summary: Fix outdated info

Reviewed By: kjchalup

Differential Revision: D38858766

fbshipit-source-id: 52b120f355e8f9e86d777875627af02c80ee45b6

Author: bottler

.github/CONTRIBUTING.md

@@ -16,14 +16,13 @@ We do not always accept new features, and we take the following factors into con
 
 When sending a PR, please ensure you complete the following steps:
 
-1. Fork the repo and create your branch from `master`. Follow the instructions
+1. Fork the repo and create your branch from `main`. Follow the instructions
    in [INSTALL.md](../INSTALL.md) to build the repo.
 2. If you've added code that should be tested, add tests.
 3. If you've changed any APIs, please update the documentation.
-4. Ensure the test suite passes:
+4. Ensure the test suite passes, by running this from the project root:
     ```
-    cd pytorch3d/tests
-    python -m unittest -v
+    python -m unittest discover -v -s tests -t .
     ```
 5. Make sure your code lints by running `dev/linter.sh` from  the project root.
 6. If a PR contains multiple orthogonal changes, split it into multiple separate PRs.