Document how to build from source

As mentioned in #253, without simple instructions on how to build
Reactime from source, the fantastic developer docs in src/README.md
are largely ineffective.

So add the steps to this developer README.  #254 already takes care
of making this README more discoverable.

Partially addresses #253.

Author: Adam Spiers

README.md

@@ -59,10 +59,15 @@ To get started, install the Reactime [extension](https://chrome.google.com/webst
 
 NOTE: The React Developer Tools [extension](https://chrome.google.com/webstore/detail/react-developer-tools/fmkadmapgofadopljbjfkapdkoienihi?hl=en) is also required for Reactime to run, if you do not already have it installed on your browser.
 
-### <b>Alternative Installation</b>
+### Installation
 
 Use `src/extension/build/build.zip` for manual installation in [Developer mode](https://developer.chrome.com/extensions/faq#faq-dev-01). Turn on 'Allow access to file URLs' in extension details page if testing locally.
 
+### Building from source
+
+Please see [the developer README](src/README.md) for instructions on building
+from source.
+
 ## <b>How to Use</b>
 
 After installing the Chrome extension, just open up your project in the browser.

src/README.md

@@ -3,6 +3,18 @@
 ## Brief
 Our mission at Reactime is to maintain and iterate constantly, but never at the expense of future developers.<br />We know how hard it is to quickly get up to speed and onboard in a new codebase.<br />So, here are some helpful pointers to help you hit the ground running. 🏃🏾💨
 
+## Building from source
+
+1. If you have already installed Reactime from the Chrome Web Store, disable or uninstall it.
+2. Run `yarn` to install all dependencies.
+3. Run `yarn dev`. This will start a `webpack` process which watches for file changes and whenever it sees some, automatically rebuilds the webpack bundles used by the extension.
+4. Go to `chrome://extensions`
+5. Ensure Developer mode is enabled
+6. Click `Load unpacked`
+7. Select the `src/extension/build` directory
+
+Now you should be able to change Reactime code and see the changes instantly reflected in your browser!
+
 ## Quick Tips
 - _Before_ beginning development, especially on teams, make sure to configure your linter and code formatting to conform to one unified setting (We recommend [the Airbnb style guide](https://github.com/airbnb/javascript)!) This will make reviewing PRs much more readable and less error-prone.