Document how to fix or disable failing CI
We want our CI to always be passing.
Sometimes, we can fix the bug quickly, or revert the commit. But if the CI environment is broken, we need to disable the failing test cases until the bug is fixed.
In doc/HACKING, document how to disable failing CI jobs. In the Travis and Appveyor configs, summarise or give an example of how to disable a failing CI config.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information