Add semantic versioning policy

[bmish]

Our policy is similar to prettier's:

If prettier produces a different output on two different versions, that's not a breaking change. Ideally prettier will never have any breaking changes, but if there are, it would be to the the API/CLI itself, not the output.

Prettier makes bug fixes and formatting tweaks to the generated output in minor/patch versions. eslint-doc-generator and prettier are both in the business of generating large amounts of content that are susceptible to a diff being triggered upon even minor bug fixes or formatting tweaks. In both cases, simply re-running the tool fixes the diff.

Also related:

• https://github.com/eslint/eslint#semantic-versioning-policy