Add additional explanations to README

Add some more explanations to the reasoning sections of various linters.
This will hopefully help better justify their existence to developers.

Also replaced some occurrences of "TBC" in the README with actual
content.

Change-Id: I2112c4dce69b3753f157df505d1f5b98f0991592
Reviewed-on: https://gerrit.causes.com/33624
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>