update docs for new RE2 mode features

Author: dlclark

README.md

@@ -43,7 +43,7 @@ The __last__ capture is embedded in each group, so `g.String()` will return the
 | Category | regexp | regexp2 |
 | --- | --- | --- |
 | Catastrophic backtracking possible | no, constant execution time guarantees | yes, if your pattern is at risk you can use the `re.MatchTimeout` field |
-| Python-style capture groups `(?P<name>re)` | yes | no |
+| Python-style capture groups `(?P<name>re)` | yes | no (yes in RE2 compat mode) |
 | .NET-style capture groups `(?<name>re)` or `(?'name're)` | no | yes |
 | comments `(?#comment)` | no | yes |
 | branch numbering reset `(?\|a\|b)` | no | no |
@@ -54,14 +54,15 @@ The __last__ capture is embedded in each group, so `g.String()` will return the
 | negative lookbehind `(?<!re)` | no | yes |
 | back reference `\1` | no | yes |
 | named back reference `\k'name'` | no | yes |
-| named ascii character class `[[:foo:]]`| yes | no |
+| named ascii character class `[[:foo:]]`| yes | no (yes in RE2 compat mode) |
 | conditionals `(?(expr)yes\|no)` | no | yes |
 
 ## RE2 compatibility mode
 The default behavior of `regexp2` is to match the .NET regexp engine, however the `RE2` option is provided to change the parsing to increase compatibility with RE2.  Using the `RE2` option when compiling a regexp will not take away any features, but will change the following behaviors:
 * add support for named ascii character classes (e.g. `[[:foo:]]`)
 * add support for python-style capture groups (e.g. `(P<name>re)`)
-
+* change singleline behavior for `$` to only match end of string (like RE2) (see [#24](https://github.com/dlclark/regexp2/issues/24))
+ 
 ```go
 re := regexp2.MustCompile(`Your RE2-compatible pattern`, regexp2.RE2)
 if isMatch, _ := re.MatchString(`Something to match`); isMatch {