Added testing instructions to devtools contributing guide

Author: Brian Vaughn

packages/react-devtools/CONTRIBUTING.md

@@ -1,3 +1,4 @@
+
 Intrested in contributing to React DevTools, but not sure where to start? This is the place!
 
 # Install project dependencies
@@ -63,6 +64,16 @@ This will launch a standalone version of Chrome with the locally built React Dev
 yarn build:chrome && yarn test:chrome --url=<url-to-test>
 ```
 
+# Unit tests
+Core DevTools functionality is typically unit tested (see [here](https://github.com/facebook/react/tree/main/packages/react-devtools-shared/src/__tests__) and [here](https://github.com/facebook/react/tree/main/packages/react-devtools-extensions/src/__tests__)). To run tests, you'll first need to build or download React and React DOM ([as explained above](#build-react-and-react-dom)) and then use the following NPM script:
+```sh
+yarn test-build-devtools
+```
+You can connect tests to a debugger as well if you'd like by running:
+```sh
+yarn debug-test-build-devtools
+```
+
 # Finding the right first issue
 The React team maintains [this list of "good first issues"](https://github.com/facebook/react/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3A%22Component%3A+Developer+Tools%22+label%3A%22good+first+issue%22) for anyone interested in contributing to DevTools. If you see one that interests you, leave a comment!