changelog: update for next release

CHANGELOG.md

@@ -1,3 +1,37 @@
+TBD
+===
+This release includes a ground-up rewrite of the regex-syntax crate, which has
+been in development for over a year.
+
+New features:
+
+* Error messages for invalid regexes have been greatly improved. You get these
+  automatically; you don't need to do anything. In addition to better
+  formatting, error messages will now explicitly call out the use of look
+  around. When regex 1.0 is released, this will happen for backreferences as
+  well.
+* Full support for intersection, difference and symmetric difference of
+  character classes. These can be used via the `&&`, `--` and `~~` binary
+  operators within classes.
+* A Unicode Level 1 conformat implementation of `\p{..}` character classes.
+  Things like `\p{scx:Hira}`, `\p{age:3.2}` or `\p{Changes_When_Casefolded}`
+  now work. All property name and value aliases are supported, and properties
+  are selected via loose matching. e.g., `\p{Greek}` is the same as
+  `\p{G r E e K}`.
+* A new `UNICODE.md` document has been added to this repository that
+  exhaustively documents support for UTS#18.
+* Empty sub-expressions are now permitted in most places. That is, `()+` is
+  now a valid regex.
+* Almost everything in regex-syntax now uses constant stack space, even when
+  performing anaylsis that requires structural induction. This reduces the risk
+  of a user provided regular expression causing a stack overflow.
+* [FEATURE #174](https://github.com/rust-lang/regex/issues/174):
+  The `Ast` type in `regex-syntax` now contains span information.
+* [FEATURE #424](https://github.com/rust-lang/regex/issues/424):
+  Support `\u`, `\u{...}`, `\U` and `\U{...}` syntax for specifying code points
+  in a regular expression.
+
+
 0.2.6 (2018-02-08)
 ==================
 Bug fixes: