Things you should know in bash additions

Includes:
* Add variable defaulting subsection
* Add 'well behaved function' point and examples to style guide
So I agree on the readability part. The issue for me is the right behavior should be chosen out of the two alternatives. In the `prefix` case we might want to pass along an empty string as the correct value, but in the server case it is unlikely to be a valid option therefore `"${1:-default.example.com}"` should be used in those cases (probably the majority of them.