docs: Add v0.6.2 release notes

[matthewfeickert]

Description

Add v0.6.2 release notes as part of completing Issue #1494.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add release notes for pyhf v0.6.2
   - c.f. https://github.com/scikit-hep/pyhf/releases/tag/v0.6.2

[codecov]

Codecov Report

Merging #1495 (02ffe20) into master (07fa272) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1495   +/-   ##
=======================================
  Coverage   97.65%   97.65%           
=======================================
  Files          63       63           
  Lines        4006     4006           
  Branches      565      565           
=======================================
  Hits         3912     3912           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.51% <ø> (ø)
unittests	97.42% <ø> (ø)

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[matthewfeickert]

Relevant page for review: https://pyhf--1495.org.readthedocs.build/en/1495/release-notes.html#release-v0-6-2

There are still more items on the v0.6.2 release checklist in Issue #1494, so we shouldn't release off this one.