Document workaround to install Chrome extension

[andreoliwa]

Summary

Add detailed explanation of how to install the Chrome extension manually.

Tests

• [ ] Chrome stable (macOS)
• [ ] Firefox stable (macOS)
• [ ] Chrome stable (Windows)
• [ ] Firefox stable (Windows)

Optional:

• [ ] Chrome beta (macOS)
• [ ] Firefox beta (macOS)
• [ ] Chrome beta (Windows)
• [ ] Firefox beta (Windows)

[yorkxin]

Thanks for your help, but I got the extension approved again :)