This commit adds more content to the readme file of the `emqx_rule_engine`
app. The aim of the changes are to make it easier to understand what the
rule engine is and what it does.
Fixes:
https://emqx.atlassian.net/browse/EMQX-9229
One step closer for better code abstraction for opensorce vs enterprise
So far this relocation is to make opensource to enterprise merges
conflict free.