Add some notes on conventions in test writing

[mikejcurry]

This PR looks to address #719 which was raised to add some guidance on test writing to the contributor guide related to conventions which have evolved over the course of cats development.

• Note, that there seems to be a mix of “type class” and “typeclass” used throughout the codebase/docs so I chose “type class”; happy to change if a decision has been made on that.
• This is far from perfect, but it’s a start and hopefully accurate.

[codecov-io]

Current coverage is 85.81%

Merging #734 into master will not affect coverage as of 4b57b9c

@@            master    #734   diff @@
======================================
  Files          162     162       
  Stmts         2249    2249       
  Branches        74      74       
  Methods          0       0       
======================================
  Hit           1930    1930       
  Partial          0       0       
  Missed         319     319       

Review entire Coverage Diff as of 4b57b9c

Powered by Codecov. Updated on successful CI builds.

[ceedubs]

Fantastic. Thanks! 👍

[stew]

this is great, thanks @mikejcurry !