Edits based on JOSS review

[iannesbitt]

Edits based on reviewer suggestions in openjournals/joss-reviews#2565, 2021-08-10.

• [x] unit tests on a CI (already implemented with Scrutinizer CI)
• [x] contributing guidelines on the github page (added here)
• [x] adding text from the Why it's special section of the docs to the paper's statement of need (added here)
• [x] compare to and reference other software (added here)
• [ ] incorporate ObsPy stream (forthcoming: #12)
• [x] clarify research usefulness (done here)
• [x] add citations (added here)
• [x] add tests for each module
  • [x] c_alert
  • [x] c_alertsound
  • [x] c_consumer
  • [x] c_custom
  • [x] c_forward
  • [x] c_plot
  • [x] c_printraw
  • [x] c_rsam
  • [x] c_telegram
  • [x] c_testing
  • [x] c_tweet
  • [x] c_write
  • [x] t_testdata
  • [x] p_producer
• [x] add code coverage

[iannesbitt]

ObsPy streaming will need to wait for a future release.

[iannesbitt]

Closing issue unless further edits are requested.