DOC: add conda-forge badge

[raybellwaves]

• Closes #xxxx
• Tests added
• Passes flake8 satpy
• Fully documented
• Add your name to AUTHORS.md if not there already

[coveralls]

Coverage increased (+0.01%) to 90.526% when pulling 8b511bd on raybellwaves:patch-2 into 480b698 on pytroll:master.

[codecov]

Codecov Report

Merging #1374 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1374   +/-   ##
=======================================
  Coverage   90.52%   90.52%           
=======================================
  Files         228      228           
  Lines       33377    33377           
=======================================
  Hits        30215    30215           
  Misses       3162     3162           

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[mraspaud]

Cool, thanks for adding the badge!