Required fixes for release

[phackstock]

Changes

There were two fixed required for building and releasing this package on pypi.

• There was an undefined rst directive in the README.rst file, causing test pypi to reject the upload.
• Because of the (side)-effect of setuptools_scm it adds all files under version control to the package. This is not desired behavior as in this case it would include all tests. This would bring the package size beyond the limit of pypi. This is where manifest.in comes in handy as it allows us to remove the tests folder from the final package. Right now I have placed all files and folders in there that seemed sensible to me, if you @jkikstra and @znicholls have suggestions for additional files that are not needed I'd be happy to add them.

Not sure if the point about tests applies in this case.
I added documentation about installation to the README.rst and the docs.

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

After merging this PR we should be good to release on pypi.

[znicholls]

lgtm, we'll learn more once we try in the wild but I think this is the right next step