Add PR template and CHANGELOG

[znicholls]

After this PR, each new PR will have a PR template to fill out (like below). Each PR also requires one approving review in order to be merged so whenever it's ready just request a review and we'll take it from there.

Pull request

Please confirm that this pull request has done the following:

• Tests added (N/A)
• Documentation added (where applicable) (N/A)
• Example added (either to an existing notebook or as a new notebook, where applicable) (N/A)
• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`#XX <https://github.com/znicholls/silicone/pull/XX>`_) Added feature which does something
- (`#XX <https://github.com/znicholls/silicone/pull/XX>`_) Fixed bug identified in (`#YY <https://github.com/znicholls/silicone/issues/YY>`_)

[codecov]

Codecov Report

Merging #1 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #1   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines           5      5           
=====================================
  Hits            5      5

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