Updating the readme for new pip install instructions

[jmwright]

This updates the readme to let users know there are two installation methods now (conda and pip), and lists the limitations of the pip installation. It also slips in a small tweak that was needed in setup.py. PyPi would not accept the long_description in the markdown format without an explicit setting to enable it.

@fpq473 you have been closely involved in getting the pip installs working, so please have a look if you get time.

[codecov]

Codecov Report

Merging #1089 (bd362d5) into master (893c799) will not change coverage.
The diff coverage is n/a.

❗ Current head bd362d5 differs from pull request most recent head cd91d57. Consider uploading reports for the commit cd91d57 to get more accurate results

@@           Coverage Diff           @@
##           master    #1089   +/-   ##
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  Coverage   96.35%   96.35%           
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  Files          40       40           
  Lines        9500     9500           
  Branches     1256     1256           
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  Hits         9154     9154           
  Misses        204      204           
  Partials      142      142           

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[fpq473]

Text looks correct and clear to me.

Minor: PyPi => PyPI; PyPi makes me think of a RaspberryPi ;)

[jmwright]

PyPi => PyPI

Fixed, thanks.

[jmwright]

@lorenzncode and @fpq473 have reviewed this and it's only a change to the readme, so I'm going to go ahead and merge.