Running fake build locally

[nojaf]

The contribution guidelines don't really mention our build pipeline that much.
People who are preparing a PR should at least locally run dotnet fake build to verify that their code works before submitting a PR.
Some people forget to format the new code for example. We should cover stuff like this in https://github.com/fsprojects/fantomas/blob/master/CONTRIBUTING.md#guidelines.