Fix the readers table in the sphinx docs so it wraps text

[djhoese]

Took the solution from here: readthedocs/sphinx_rtd_theme#117
And walked through here: https://rackerlabs.github.io/docs-rackspace/tools/rtd-tables.html

Basically the default RTD theme doesn't wrap table text which was making our table of readers on the home page look really bad.

[coveralls]

Coverage remained the same at 73.078% when pulling e26d64c on djhoese:bugfix-docs-table into f51790b on pytroll:master.

[codecov]

Codecov Report

Merging #444 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #444   +/-   ##
=======================================
  Coverage   73.07%   73.07%           
=======================================
  Files         134      134           
  Lines       17662    17662           
=======================================
  Hits        12907    12907           
  Misses       4755     4755

Impacted Files	Coverage Δ
satpy/writers/__init__.py	75.69% <ø> (ø)	⬆️

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