Document how CI works

[friism]

Appveyor is reporting that my PR is not good: https://ci.appveyor.com/project/PowerShell/openssh-portable

It's unclear to me how the CI even works, the Appveyor artifacts don't seem to be in the default branch. So it's hard to figure out how to fix it.

cc @bingbing8

[manojampalam]

We are working through other issues that are breaking the validation system. Will put out a document on how the CI system works once that is done.