Mocha guidance

[andyw8]

To start off the Mocha section (#37) I would propose these as recommended configuration settings:

 Mocha.configure do |c|
   c.stubbing_method_on_nil = :prevent
   c.stubbing_non_existent_method = :prevent
   c.stubbing_non_public_method = :prevent
 end

The first two can prevent subtle bugs which may cause false passes.

The third encourages better design by avoiding tests having knowledge of private implementation details.

https://mocha.jamesmead.org/Mocha/Configuration.html

[andyw8]

Note that stubbing_method_on_nil is now deprecated:

Mocha::Configuration#stubbing_method_on_nil=` is deprecated and will be removed in a future release. `nil` is frozen in Ruby >= v2.2 and Mocha will be dropping support for Ruby v2.1. At that point it won't be possible to stub methods on `nil` any more.