Add Ripgrep config

[ootoovak]

It is another alternative to Ack. But perhaps just a list of alternatives would do and one example?

[auscompgeek]

This is duplicating documentation… one section in the README would be enough, would it not?

[peteraba]

Mentioning ripgrep would still be useful I think.

[ZeroKnight]

Perhaps just change this heading to something like:

#### Can I use `ag` ([The Silver Searcher]), `rg` ([Ripgrep]) or similar with this?

This avoids needlessly duplicating the documentation and gets the same message across.