CHECKSRC.md: clarified, explained the whitelist file

## What does checksrc warn for?

checksrc does check and verify the entire style guide, but the script is an

effort to detect the most common mistakes and syntax mistakes that

contributers make before they get accustomed to our code style. Heck, many of

us regulars do the mistakes too and this script helps us keep the code in

shape.

checksrc does not check and verify the code against the entire style guide,

but the script is instead an effort to detect the most common mistakes and

syntax mistakes that contributers make before they get accustomed to our code

style. Heck, many of us regulars do the mistakes too and this script helps us

keep the code in shape.

    checksrc.pl -h

### Directory wide ignore patterns

`checksrc.whitelist` describe!

This is a method we've transitioned away from. Use inline ignores as far as

possible.

Make a `checksrc.whitelist` file in the directory of the source code with the

false positive, and include the full offending line into this file.