Add instructions for Chrome

[multimeric]

Although they removed this from the Chrome store, I had no issues actually installing it from the releases page, using the following steps:

• Download the source code of the latest release from the releases page
• Untar/zip the archive
• Browse to chrome://extensions/ in chrome
• Click "Load unpacked" at the top left
• Browse to /path/to/ProtonDB-for-Steam-X.X.X/src, and click okay
• Ignore the warning about Unrecognized manifest key 'applications'

It might be helpful to add this to the readme for people using Chrome.

[tryton-vanmeer]

I was providing a CRX file, but I kinda got lazy about that. I'll add these instructions to the README.

I should also add a note about possible breaking changes, and for people to make issues if Chrome specific issues pop-up.

[multimeric]

Cheers, that sounds good!