arachni: adding README.md

# arachni

https://www.arachni-scanner.com/

Arachni is a feature-full, modular, high-performance Ruby framework aimed

towards helping penetration testers and administrators evaluate the security of

modern web applications.

It is versatile enough to cover a great deal of use cases, ranging from a

simple command line scanner utility, to a global high performance grid of

scanners, to a Ruby library allowing for scripted audits, to a multi-user

multi-scan web collaboration platform. In addition, its simple REST API makes

integration a cinch.