Automatically add .github directory to .Rbuildignore

[hezibu]

First of all, many thanks for the package - new version works well and I love the use of github actions.

I've encountered the following NOTE using rhub::rhub_check():

Found the following hidden files and directories:
  .github
These were most likely included in error. See section 'Package
structure' in the 'Writing R Extensions' manual.

CRAN-pack does not know about
  .github

Adding .github to .Rbuildignore eliminates this NOTE. I was wondering if rhub::rhub_setup() should automatically add .github to .Rbuildignore or throw a message in the console to do so prior to using rhub::rhub_check() in case it is not added yet.

Thanks