Document Android / mobile development process

[cowlicks]

A good starting point is #1516 (comment) and #1516 (comment)

[andresbase]

We also need to figure out how to test on mobile for different versions and screens that doesn't take too long. I'll ask E&D but any recommendations are welcome.

[cowlicks]

Right now, most of the code for mobile can be tested with unittests. We can probably test UI stuff just fine by forcing the window size to have mobile dimensions on travis-ci + mocking firefoxandroid.isUsed.

I don't think we need something as elaborate as saucelabs, at least not yet, since we are not testing a full mobile app.

[ghostwords]

web-ext now supports testing Firefox extensions for Android; see mozilla/web-ext#737 (comment) and this demo video.

We may not need to do anything beyond linking to https://extensionworkshop.com/documentation/develop/getting-started-with-web-ext/#testing-in-firefox-for-android somewhere in our documentation (the developer guide?).

[ghostwords]

Regarding testing, see #1624.