(MODULES-927) Update readme

Fixes https://tickets.puppetlabs.com/browse/MODULES-927.

Add missing functions to README:

* anchor
* bool2str
* camelcase
* deep_merge
* pick_default
* validate_ipv4_address
* validate_ipv6_address

Update docs for functions in README:

* range (take an optional third argument for range step)
* validate_slength (take an optional third argument for minimum length)
* file_line (take after and multiple attributes)

Resolves #316 
Resolves #271