Update CONTRIBUTING.md on bugfixes/features PRs
📚 Documentation
I might've missed the context, but we recently changed the strategy of which branches feature/bugfix PRs should get merged into, didn't we? If so, the docs should reflect it.

  
    
      pytorch-lightning/.github/CONTRIBUTING.md
    
    
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           _Note, that this flow may change after the 1.2 release as we will adjust releasing strategy._