(MODULES-927) Add missing functions to README

* anchor
* bool2str
* camelcase
* deep_merge
* pick_default
* validate_ipv4_address
* validate_ipv6_address
(MODULES-927) 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)
Merge pull request #302 from 3flex/update-readme

(MODULES-927) Update readme