Update CONTRIBUTING.md (#6061)

Author: pshrmn

CONTRIBUTING.md

@@ -8,6 +8,7 @@ Please read https://reactjs.org/ and the Code of Conduct before opening an issue
 - [Proposing New or Changed API?](#api)
 - [Issue Not Getting Attention?](#attention)
 - [Making a Pull Request?](#pr)
+- [Setup](#setup)
 - [Development](#development)
 - [Hacking](#hacking)
 
@@ -45,10 +46,48 @@ All commits that change or add to the API must be done in a pull request that al
 - Updates examples
 - Updates the docs
 
+## Setup
+
+The following steps will get you setup to contribute changes to this repo:
+
+1. Fork the repo (click the <kbd>Fork</kbd> button at the top right of this page).
+2. Clone your fork locally.
+```bash
+# in a terminal, cd to parent directory where you want your clone to be, then
+git clone https://github.com/<your_github_username>/react-router.git
+cd react-router
+```
+3. Install dependencies and build. React Router uses `npm`, so you should too. If you install using `yarn`, unnecessary yarn lock files will be generated.
+```bash
+npm install
+npm run build
+```
+
 ## Development
 
-- `npm test` starts a karma test runner and watch for changes
-- `npm start` starts a webpack dev server that will watch for changes and build the examples
+### Packages
+
+React Router uses a monorepo to host code for multiple packages. These packages live in the `packages` directory.
+
+### Testing
+
+Calling `npm test` from the root directory will run **every** package's tests. If you want to run tests for a specific package, you should `cd` into that directory.
+```bash
+# all tests
+npm test
+# react-router-dom tests
+cd packages/react-router-dom
+npm test
+```
+React Router uses Jest to run its tests, so you can provide the `--watch` flag to automatically re-run tests when files change.
+
+### Website
+
+The code for the documentation website lives in the `website` directory. `cd` into there and call `npm start` to start a webpack dev server on `localhost:8080` that will watch for changes.
+```bash
+cd website
+npm start
+```
 
 ## Hacking