Note more languages' package sites.

Change-Id: Ia333c359182283f3654a66df5fbab6809c11e52d
Reviewed-on: https://code-review.googlesource.com/25170
Reviewed-by: Paul Wankadia <junyer@google.com>

README

diff --git a/README b/README
index 5d8d2ae..681d6d7 100644
--- a/README
+++ b/README
@@ -28,11 +28,11 @@
 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 Erlang wrapper is at https://github.com/tuncer/re2/ and on Hex (hex.pm).
 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.
-An OCaml wrapper is at https://github.com/janestreet/re2/ and on OPAM.
-A Perl wrapper is at https://github.com/dgl/re-engine-RE2/ and on CPAN.
-A Python wrapper is at https://github.com/facebook/pyre2/.
-An R wrapper is at https://github.com/qinwf/re2r/.
-A Ruby wrapper is at https://github.com/mudge/re2/.
+A Node.js wrapper is at https://github.com/uhop/node-re2/ and on NPM (npmjs.com).
+An OCaml wrapper is at https://github.com/janestreet/re2/ and on OPAM (opam.ocaml.org).
+A Perl wrapper is at https://github.com/dgl/re-engine-RE2/ and on CPAN (cpan.org).
+A Python wrapper is at https://github.com/facebook/pyre2/ and on PyPI (pypi.org).
+An R wrapper is at https://github.com/qinwf/re2r/ and on CRAN (cran.r-project.org).
+A Ruby wrapper is at https://github.com/mudge/re2/ and on RubyGems (rubygems.org).