Add link to external guidelines for commit messages, changelogs

[benlk]

This section doesn't need to be formal INN standards, but it would be nice to have some resources in INN/docs for formatting of commit messages and changeloge. The documentation at https://github.com/INN/docs/blob/master/how-we-work/version-control.md is very "how to use git" and could be extended with commit message and changelog guidelines.

Things to consider:

• linking to good INN pull requests/commits/changelogs/release notes
• linking to external standards
• improving INN's own GitHub pull request boilerplate and contributing.md boilerplate

[benlk]

Commit message formatting, from AngularJS: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit#

Changelog formatting: https://keepachangelog.com/en/1.0.0/