README: add reference to libcre2 C binding for RE2

Change-Id: I3cb36fbc261789968b5a64000cd4656622fa3f83
Reviewed-on: https://code-review.googlesource.com/4100
Reviewed-by: Paul Wankadia <junyer@google.com>

README

diff --git a/README b/README
index 2c23660..bbe6dc1 100644
--- a/README
+++ b/README
@@ -23,6 +23,8 @@
 under the BSD-style license found in the LICENSE file.
 
 RE2's native language is C++.
+
+A C wrapper is at https://github.com/marcomaggi/cre2/.
 An Erlang wrapper is at https://github.com/tuncer/re2/.
 An Inferno wrapper is at https://github.com/powerman/inferno-re2/.
 A Node.js wrapper is at https://github.com/uhop/node-re2/ and on NPM.