Explain how to run C++ tests locally in CONTRIBUTING.md

Add breadcrumb to our script for running C++ tests locally.

Differential Revision: [D67957088](https://our.internmc.facebook.com/intern/diff/D67957088/)

[ghstack-poisoned]

Author: swolchok

CONTRIBUTING.md

@@ -197,7 +197,7 @@ If it's not clear how to add a test for your PR, take a look at the blame for
 the code you're modifying and find an author who has more context. Ask them
 for their help in the PR comments.
 
-TODO: Explain how to run tests locally without needing to push and wait for CI.
+The `test/run_oss_cpp_tests.sh` script will build and run C++ tests locally.
 
 ### Continuous Integration
 See https://hud.pytorch.org/hud/pytorch/executorch/main for the current state of