Document CircleCI recommendations

[Peter-Darton-i2]

For: #37

As described in #37, the documentation has a hole where the CircleCI recommendations should be.

CircleCI support are recommending this logger to CircleCI users (like me), so it's time we plugged that hole in the documentation.

Note: No functional changes in here, just documentation.

[Siphonophora]

@Peter-Darton-i2 Thanks for writing this up. Much appreciated!

[Peter-Darton-i2]

I've made a few changes; I think I've addressed all the previous review comments, and incorporated some changes CircleCI requested too.

I think this is ready for merge.

[Siphonophora]

@Peter-Darton-i2 Thanks for the contribution