Add change log markdown file

[nathanshammah]

Description

Add CHANGELOG.md in the root directory, including it in the top bar menu of the documentation. Ticks a box in #268.

Checklist

Please make sure you have finished the following tasks before requesting a review of the pull request (PR). For more information, please refer to the Contributor's Guide:

• Contributions to mitiq should follow the pep8 style. You can enforce it easily with black style and with flake8 conventions.
• Please add tests to cover your changes, if applicable, and make sure that all new and existing tests pass locally.
• If the behavior of the code has changed or new feature has been added, please also update the documentation.
• Functions and classes have useful Google-style docstrings and type hints in the signature of the objects.
• The changelog is updated, including author and PR number (@username, gh-xxx).

[codecov]

Codecov Report

Merging #328 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #328   +/-   ##
=======================================
  Coverage   91.77%   91.77%           
=======================================
  Files          17       17           
  Lines         827      827           
=======================================
  Hits          759      759           
  Misses         68       68           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 372d9f8...c175e72. Read the comment docs.