Link to Troubleshooting section in README for users

[brandon-leapyear]

In Checks tab and/or Mergit website.

It should be a permanent piece of text, such that it's visible if there are issues. Probably not just the Checks tab, in case of https://github.com/LeapYear/mergit#i-pushed-a-commit-but-nothing-is-showing-up-in-the-checks-tab