Provide guidance around specific pull request description formatting for breaking change notes

[dgutride]

We'd like to extract breaking change notes related to developer impact from pull requests. Please provide a specific format that can be used to pull text from the description of a PR (wrapping in tildes for example - Rename pf-var-one to pf-var-two). This should allow us to run a script against github pr's regularly during breaking change release cycles to provide a digest of information for consumers that might want to be proactively keeping up.

[evwilkin]

Please use the following format at the bottom of the PR description for notating any breaking change PRs (as demonstrated here and here):

## Breaking changes
1. Any **md content** here
  - More content

Breaking changes

1. Any md content here

• More content

[redallen]

Our breaking change release is now over. This format worked well and I'd recommend it again!