Update custom operators documentation

[K-Phoen]

See #38 and #40

[bpolaszek]

+1 😄

I need to create an in_ip_range operator to check that kind of rule:

ip in_ip_range 192.168.1.*

I can validate this rule with a callable based on mlocati/ip-lib library, but I don't figure out how to register that callable in RulerZ.
I can't find any ArrayExecutor class or ExtendableExecutor interface as referred to in the documentation, and anyway, the documentation does not indicate how to register it in RulerZ.

How can I achieve this?