Add ppx_regexp.descr.

ppx_regexp.descr

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+Matching Regular Expressions with OCaml Patterns
+
+This syntax extension turns
+
+    match%pcre x with
+    | {|re1|} -> e1
+    ...
+    | {|reN|} -> eN
+    | _ -> e0
+
+into suitable invocations to the ocaml-re library.  The patterns are plain
+strings of the form accepted by `Re_pcre`, except groups can be bound to
+variables using the syntax `(?<var>...)`.  The type of `var` will be
+`string` if a match is of the groups is guaranteed given a match of the
+whole pattern, and `string option` if the variable is bound to or nested
+below an optionally matched group.