README mentioned repos for wrappers for Erlang, Inferno, Python and Ruby.  There is
also a Perl wrapper for the RE2 engine on CPAN, and it has a github repository.
This patch adds mention of the Perl wrapper, provides a link to the GitHub
repository (which seems in-keeping with how other languages have done), and then
additionally mentions that it is also on CPAN -- where Perl users would expect
to find it.

Change-Id: Ic796543012dbd57817d9c0c91ad41c17958fc1cf
Reviewed-on: https://code-review.googlesource.com/2460


Reviewed-by: Paul Wankadia <junyer@google.com>