Add contributing info

Author: timlegge

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+# Contributing
+
+The best was to contribute is to fork https://github.com/perl-net-saml2/perl-Net-SAML2 and send a pull request.
+
+Issues should be reported at [Github Issue](https://github.com/perl-net-saml2/perl-Net-SAML2/issues).
+
+Future Plans:
+
+1. Move to configurable Certificate Verification with Crypt::OpenSSL::Verify replacing Crypt::OpenSSL::VerifyX509 as the default eventually
+2. Allow XML Signature Verification to be configurable with XML::Sig possibly replacing Net::SAML2::XML::Sig at the default eventually
+3. Allow the certificate files to be provided as text strings to allow applications to store the certificates in their native settings if needed
+4. We are open to other suggestions or improvements