How to work on a feature branch that relies on another unmerged feature branch?
Branching featureB off featureA is the best approach. Rather than monitoring when featureA is approved, submit the pull request when you're ready and note in the request that it depends on featureA. Reference the issue number in the pull request and Github will place notes itself. Something like "this depends on #123 being accepted".
If featureA is changed before being accepted, you (or the person accepting the pull request) should merge or rebase with featureA prior to merging featureB.