Add conda install information

[znicholls]

• [x] Tests added (N/A)
• [x] Documentation added
• [x] Example added (in the documentation, to an existing notebook, or in a new notebook) (N/A)
• [x] Description in CHANGELOG.rst added (single line such as: (`#XX <https://github.com/openscm/openscm-twolayermodel/pull/XX>`_) Added feature which does something)

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@a2f78d6). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #33   +/-   ##
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  Coverage          ?   97.66%           
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  Files             ?        7           
  Lines             ?      386           
  Branches          ?       23           
=========================================
  Hits              ?      377           
  Misses            ?        6           
  Partials          ?        3           

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