Add initial CONTRIBUTING.md

Author: Mark Haine

CONTRIBUTING.md

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+# How to contribute
+
+I'm really glad you're reading this, because we need volunteer developers to help this effort come to fruition.
+
+Here are some important resources:
+
+  * [Digital Credentials Protocols (DCP) Working Group Home Page](https://openid.net/wg/digital-credentials-protocols/) should give you a little background
+  * Mailing list: Join our [mailing list]([[email protected]](http://lists.openid.net/mailman/listinfo/openid-specs-digital-credentials-protocols)) to ask questions or provide feedback
+  * Bugs? the [issue tracker](/openid/OpenID4VCI/issues) is where to report them
+
+## Intellectual Policy
+
+You have to sign IPR Contribution Agreement to file issues. [More info on IPR](http://openid.net/intellectual-property/)
+
+Thanks,
+DCP Working Group Co-chairs