Add `README.rst` to `MANIFEST.in`

[dhirschfeld]

With the addition of exclude * to the MANIFEST.in, the README.rst is no longer being included in the sdist: xref: https://github.com/conda-forge/pgspecial-feedstock/pull/16#issuecomment-778549766

Without that file the setup.py fails and the package can't be built from the sdist: https://github.com/dbcli/pgspecial/blob/d3e03b74b52be823f5d64adf9eb1894a56be7cbf/setup.py#L24

This should ensure the README.rst file is included in the sdist.

[dhirschfeld]

Fixes #101

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@06af436). Click here to learn what that means. The diff coverage is 13.17%.

@@           Coverage Diff            @@
##             master    #102   +/-   ##
========================================
  Coverage          ?   8.51%           
========================================
  Files             ?       6           
  Lines             ?    2466           
  Branches          ?       0           
========================================
  Hits              ?     210           
  Misses            ?    2256           
  Partials          ?       0           

Impacted Files	Coverage Δ
pgspecial/help/commands.py	0.20% <ø> (ø)
pgspecial/dbcommands.py	2.42% <5.38%> (ø)
pgspecial/iocommands.py	26.44% <37.93%> (ø)
pgspecial/main.py	31.10% <51.11%> (ø)
pgspecial/namedqueries.py	92.00% <75.00%> (ø)
pgspecial/__init__.py	100.00% <100.00%> (ø)

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[j-bennet]

🚒 Great, thanks a lot!

[j-bennet]

..and released.