add changelog and config file

[NathanPJF]

What are you trying to accomplish with this PR?

Add a changelog to the repo. Based off the same changelog process we're using for slate-tools: https://github.com/Shopify/slate-tools/blob/master/CHANGELOG.md

Checklist

For contributors:

• I have updated the documentation in the README file to reflect these changes, if applicable.

For maintainers:

• I have 🎩'd these changes.
• I have bumped the package.json version in a separate PR, if applicable.

Considerations:

• These changes will require the public Slate docs to be updated. See the Shopify/slate repo for notes on updating docs, if applicable.

[macdonaldr93]

I wonder if you could automate this with ship it.