full text search of ".txt", ".md", and similar files

This is not a bug report nor a feature request so I hope you'll forgive me for using this format.

I'm sure there must be a simple way to implement full text search, but I've not been able to figure it out from the documentation. I just want to type in one or more strings in some box and get back a list of text files (.txt, .md, etc) that contain those strings.

I was also not able to find some other suitable forum in which to ask this question. Any help is appreciated. Thanks.