Update CONTRIBUTING.md

[ericmarkmartin]

Added a bullet to the style guide to use single quotes.

[coveralls]

Coverage remained the same at 82.662% when pulling 582ff88 on ericmarkmartin:patch-4 into f29b1cb on nightscout:master.

[codecov-io]

Current coverage is 82.66%

Merging #1592 into master will not affect coverage as of 603bab5

@@            master   #1592   diff @@
======================================
  Files           79      79       
  Stmts         6108    6108       
  Branches         0       0       
  Methods          0       0       
======================================
  Hit           5049    5049       
  Partial          0       0       
  Missed        1059    1059       

Review entire Coverage Diff as of 603bab5

---

Uncovered Suggestions

1. +0.19% via .../js/profileeditor.js#324...335
2. +0.18% via .../js/profileeditor.js#269...279
3. +0.16% via .../report/js/report.js#259...268
4. See 7 more...

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