Improve documentation

[jaitaiwan]

Hi Folks,

I've come here from the go-ruleguard package in an attempt to understand beyond the examples how the gogrep part of go-ruleguard works.

Unfortunately there's not really much information to help me with what to do and where to go (unless there is and someone can point me to it).

Thanks

[vendelin8]

https://go-ruleguard.github.io/by-example