Add changelog file

[jklenzing]

Description

Adds CHANGELOG.md and requires pull requests to summarize changes there.

Type of change

• This change is a documentation update

Checklist:

• Make sure you are merging into the develop (not master) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[JonathonMSmith]

Is this TBD format what should be used when submitting a pull request?

[jklenzing]

My plan is for the TBD to be updated as part of the pull to master, since that is when the version will be applied. Probably a more elegant way of doing this, but it works for now.