PostGIS development planning happens via a Trac instance.
In order for proper scheduling of contributions acceptance/merging it is recommended to file a ticket there and add your patch or a link to a patch or to a public git branch where your development is ongoing.
Filing a ticket requires setting up an OSGeo account, which lets you contribute to every project of the Open Source Geospatial Foundation.
As we understand you might not want to setup a new account, simply filing a pull request on one of the PostGIS code mirrors is also welcome, but it is easier for such contributions to miss our radars unless it also has a matching Trac ticket.
If you intend to be involved for more than an occasional patch we recommend you to subscribe to the development mailing list.
Note that We also welcome git patches via email