Create "Get it on Accrescent" button

[lberrymage]

Ideally when clicked, the button link should redirect to the corresponding app page in the Accrescent client. If Accrescent is not installed, then the user should be redirected to a page instructing them to install Accrescent first. We don't have a web UI like Google Play and F-Droid (and I have no immediate plans for one), so we can't direct the user to a web page for the app.

To-do:

• [x] https://github.com/accrescent/accrescent/issues/5
• [x] Create button graphic
• [x] Publish to website
• [x] https://github.com/accrescent/accrescent/issues/19
• [x] Add website redirect to installation page for when Accrescent is not installed
• [x] Add badge generator to developer console
• [x] Document usage

[lberrymage]

Badge added to website in https://github.com/accrescent/accrescent.app/pull/8