Add license

[znicholls]

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>`_)

Closes #6

[codecov]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #12   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines          69     69           
  Branches        8      8           
=====================================
  Hits           69     69

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