Add CHANGELOG.md file

[tilo]

Problem

Users can not easily discover what changes were made in Rbi, because this project does not have a CHANGELOG.md file.

Proposed solution

Add a CHANGELOG.md file that lists each released version and details the changes that were made.
This is a standard in Ruby Gems, and will automatically be picked up by rubygems.org and other sites

[andyw8]

You can find details of the changes on the releases page:

https://github.com/Shopify/rbi/releases