Document CI setup (AppVeyor/Codecov/Travis)

[fingolfin]

We should document out test setup somwhere. E.g. by adding some helpful explanatory comments to etc/ci.sh, and/or in a Wiki page, and/or in a text document in the doc/ dir or whatever.

As it is, right now if anything were to break regarding Travis, AppVeyor, Codecov, the only thing one can do is to email the mailing list and/or directly Alex, Markus, Chris, and hope one of them is able to fix it.

Based on past experiences in other OSS projects, I think this is somewhat problematic. For example, if nobody touches the setup for half a year (because everything works great -- yay!), they may have forgotten some steps. And if people for some reasons are not available, somebody else needs to "reverse engineer" it.

This documentation need not be very long and complex, to the contrary. A summary of how the integration works, where one can find the relevant settings etc. is enough.

UPDATE: some concrete things that should be documented:

• what CI services we use (Travis, AppVeyor; more in the future?)
  • how to run these tests locally (see also issue Target citests in Makefile? #1740)
  • what about Jenkins??
• what coverage services we use (Codecov, Coveralls)
  • how to
• how it all works (discuss .travis.yml, stuff in etc/ ?)
• admin information:
  • how to give people access to AppVeyor so that they can restart or cancel jobs?

But I think instead of writing everything down one might think of, it's best to do this: take some helpful person who wants to learn about these things; let them start looking for themselves (with access to the documentation of Traivs and all). Then sit next or near to them, and encourage them to ask questions if they get stuck. Use this to decide what kinds of pointers are helpful, then just write those, and for the rest, rely on the service's existing information (to which we may also link, of course)

[fingolfin]

While this would be nice to have, I don't think it's mandatory for 4.9.0, hence I just removed the milestone. Perhaps we can try to get this into 4.10.0.