[QAOA] Documents Changes

[Lucaman99]

Context:

Part of the ongoing effort to add QAOA functionality to PennyLane

Description of the Change:

Adds all of the new QAOA features to CHANGELOG.md

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

Merging #754 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #754   +/-   ##
=======================================
  Coverage   95.64%   95.64%           
=======================================
  Files         113      113           
  Lines        6981     6981           
=======================================
  Hits         6677     6677           
  Misses        304      304           

Impacted Files	Coverage Δ
pennylane/qaoa/cost.py	100.00% <ø> (ø)
pennylane/qaoa/mixers.py	100.00% <ø> (ø)

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