Handling PRs that update the platform interface

[ryanheise]

Documentation request for the following page(s)

https://github.com/ryanheise/audio_service/blob/minor/CONTRIBUTING.md

Section(s) in question

Making a pull request

Suggested improvement

Changes to the platform interface need to be published in stages, and yet the major and minor branches should always be in a releasable state.

I think the only way out of this is to do pull requests like this as two separate pull requests, one for the platform interface changes and the other for the main plugin changes. The latter could branch off the former.

We might need a similar branching approach for major changes to the web implementation.