make it obvious that forking is easy

[cben]

Meta issue for lower barrier to entry:

• [ ] "fork this on github" or similar
• [ ] CONTRIBUTING.md
• [x] easy deploy - #76
• [ ] an easy way to at least provide feedback without github account

[cben]

Configured issue submission without GH user at https://gitreports.com/issue/cben/mathdown

Not yet linked from UI, not sure how I'd like top bar to look. Given enough space, probably something like this:

[:speech_balloon: feedback] [:octocat: dev]

In ideal world, [dev] leads to (1) github or CONTRIBUTING.md (emphasis on ease of running a fork), (2) issues (3) current version, test results etc (4) git graph of recent changes (GH network?). In ideal² world, one can click any version to use it.
In practice, [dev] -> CONTRIBUTING.md is OK as entry point for power users.

[feedback] ideally lets one type within the app but link to Gitreports (which links power users to github issues) is OK as entry point.

Given less space, e.g. mobile, I'd have :octocat: icon only + [feedback] button leading to Gitreports (which again can link to github).

[cben]

• [ ] Provide link (and/or header?) to the exact source version that is running.
  This would IIUC be a legal requirement if I were using Affero GPL; I'm not but it is a desired quality in hosted free software.