FIX: Add mention to potential alternatives in JOSS submission

[oesteban]

Re: openjournals/joss-reviews#3459 (comment)

[codecov-commenter]

Codecov Report

Merging #132 (f8f634d) into master (24aac2c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #132   +/-   ##
=======================================
  Coverage   98.88%   98.88%           
=======================================
  Files          12       12           
  Lines        1081     1081           
  Branches      138      138           
=======================================
  Hits         1069     1069           
  Misses          6        6           
  Partials        6        6           

Flag	Coverage Δ
travis	98.88% <ø> (ø)

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