CONTRIBUTING: Explain routes of contributing

Attempt to explain the 3 routes the ports project has for contribution.

Reviewed by:		pauamma@gundo.com, mhorne, fernape
Approved by:		fernape (ports), mhorne (mentor)
Differential Revision:	https://reviews.freebsd.org/D47169

CONTRIBUTING.md

@@ -1,3 +1,9 @@
-FreeBSD does not currently accept pull requests. This is a read-only mirror from SVN.
+# FreeBSD Ports Collection Contribution Guidelines
 
-Please see the [Porter's Handbook](https://docs.freebsd.org/en/books/porters-handbook/) section on [Submitting patches](https://docs.freebsd.org/en/books/porters-handbook/quick-porting/#porting-submitting).
+Contributions may be submitted to the FreeBSD Ports Collection via one of three methods:
+
+- Attachments to [Bugzilla tickets](https://bugs.freebsd.org) are preferred, as this notifies existing maintainers.
+- Code reviews on [Phabricator](https://reviews.freebsd.org/differential) are accepted by some maintainers.
+- GitHub Pull Requests are an experimental feature and should expect additional delay.
+
+To learn more about contributing to the FreeBSD Ports Collection, please see the [Porter's Handbook](https://docs.freebsd.org/en/books/porters-handbook/) section on [Submitting patches](https://docs.freebsd.org/en/books/porters-handbook/quick-porting/#porting-submitting).