[Gatsby] README.md instructions (#10945)

* Padded out README.md instructions

* fixed markdown links and added reloading notes

* added more yarn instructions

* removed html.js instruction

Author: joecritch

www/README.md

@@ -1,16 +1,55 @@
-# React documentation site
-
-To get setup:
-```
-yarn install
-```
-
-To run the dev server:
-```
-yarn run dev
-```
-
-To clear the local cache:
-```
-yarn run reset
-```
+# React — documentation site
+
+## Getting started
+
+### Prerequisites
+
+1. Git
+1. Node: install version 7.6 or greater
+1. Yarn: `npm i -g yarn` to install it globally via NPM
+1. A clone of the [React repo](https://github.com/facebook/react) on your local machine
+1. A fork of the repo (for any contributions)
+
+### Installation
+
+1. `cd react` to go into the project root
+1. `yarn` to install the main NPM dependencies
+1. `cd www` to go into the website project
+1. `yarn` to install the website's NPM dependencies
+
+### Running locally
+
+1. `yarn dev` to start the hot-reloading development server powered by Gatsby
+1. `open http://localhost:8000` to open the site in your favourite browser
+
+## Contributing
+
+### Create a branch
+
+1. `git checkout master` from any folder in your local react repository
+1. `git pull origin master` to ensure you have the latest main code
+1. `git checkout -b wwwTheNameOfMyChange` (replacing `TheNameOfMyChange` with a suitable name) to create a branch
+
+### Make the change
+
+1. Follow the "Running locally" instructions
+1. Save the files and check in the browser
+  1. Any React components in `src/components` or `src/templates` will hot-reload
+  1. If working with plugins, you may need to remove the `.cache` directory and restart the server
+
+### Test the change
+
+1. If possible, test any visual changes in all latest versions of common browsers, on both desktop and mobile.
+1. Run `yarn prettier-all && yarn lint` from the project root (outside of `www`)
+
+### Push it
+
+1. `git add -A && git commit -m "My message"` (replacing `My message` with a commit message, such as `Fixed header logo on Android`) to stage and commit your changes
+1. `git push yourname wwwTheNameOfMyChange`
+1. Go to the [React repo](https://github.com/facebook/react) and you should see recently pushed branches.
+1. ... follow GitHub's instructions...
+1. If possible, include any screenshots of visual changes. A Netlify build will also be automatically created once you make your PR, so other people can see your change.
+
+## Troubleshooting
+
+- `yarn reset` to clear the local cache