add regex replace to readme

This is the last ticket to resolve #378

Author: yshavit

README.md

@@ -131,11 +131,13 @@ In any of the above, the text may be:
 - an `unquoted string` that starts with a letter; this is case-insensitive
 - a `"quoted string"` (either single or double quotes); this is case-sensitive
 - a string (quoted or unquoted) anchored by `^` or `$` (for start and end of string, respectively)
-- a `/regex/`
+- a `/regex/` match or `!s/regex/replace/` (see the user manual for [caveats about replacements])
 - omitted or `*`, to mean "any"
 
 See the [tutorial] for a bit more detail, and [user manual] for the full picture.
 
+[caveats about replacements]: https://github.com/yshavit/mdq/wiki/Full-User-Manual#regex-replacements
+
 [tutorial]: https://github.com/yshavit/mdq/wiki/Tutorial
 
 [user manual]: https://github.com/yshavit/mdq/wiki/Full-User-Manual