SedRegex: add configuration / usage instructions

From: 9d0dce6ef5

plugins/SedRegex/README.md

@@ -1 +1,25 @@
 History replacer using sed-style expressions.
+
+### Configuration
+
+Enable SedRegex on the desired channels: `config channel #yourchannel plugins.sedregex.enable True`
+
+### Usage
+
+After enabling SedRegex, typing a regex in the form `s/text/replacement/` will make the bot announce replacements.
+
+```
+20:24 <~GL> helli world
+20:24 <~GL> s/i/o/
+20:24 <@Lily> GL meant to say: hello world
+```
+
+You can also do `othernick: s/text/replacement/` to only replace messages from a certain user. Supybot ignores are respected by the plugin, and messages from ignored users will only be considered if their nick is explicitly given.
+
+#### Regex flags
+
+The following regex flags (i.e. the `g` in `s/abc/def/g`, etc.) are supported:
+
+- `i`: case insensitive replacement
+- `g`: replace all occurences of the original text
+- `s`: replace only messages from the caller