Add install instructions to README

[sosnovsky]

Add instructions on how to install development build to Chrome and Firefox

chrome instructions from #5007 (comment)

Please follow those steps.

1. Open your Chrome browser and navigate to chrome://extensions/.
2. If not already enabled, toggle on the Developer mode switch located at the top-right corner.
3. Click on the Load Unpacked button.
4. Browse to and select the appropriate folder, either build/chrome-consumer or chrome-enterprise.
Once completed, the extension will be successfully installed and ready to use.