Issues are now on GitHub (#315)

This removes the Trac Ticket instructions and adds GitHub issues.
The link goes to templates like the New issue button on GitHub.
The old issues on Trac are not mentioned for simplicity
and it is up to the community (not the reporter)
to identify and migrate any duplicates.

CONTRIBUTING.md

@@ -7,10 +7,11 @@ https://grass.osgeo.org/get-involved/.
 In the rest of this document, we will focus on contributions centered
 around the GRASS GIS source code.
 
-## Reporting issues and suggesting features and changes
+## Reporting issues and suggesting features
 
-* Get an OSGeo Services UserID (http://www.osgeo.org/osgeo_userid).
-* Got to https://trac.osgeo.org/grass/#BugTracking and follow instructions there.
+To report an issue or to suggest features or a change,
+[open an issue](https://github.com/OSGeo/grass/issues/new/choose)
+on GitHub.
 
 ## Changing code and documentation