remove trac references, promise details soon

Author: dimpase

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,4 @@
-Thanks for contributing to Sage! Unfortunately we are not accepting pull
-requests on GitHub yet. To propose a change to Sage, please log in with your
-GitHub account at https://trac.sagemath.org, create a ticket, and push your
-changes to that ticket as explained in our
-[Developer's Guide](https://doc.sagemath.org/html/en/developer/manual_git.html).
+Thanks for contributing to Sage! Detailed instructions to be added shortly.
 
 Please make sure to also have a look at our
 [Code Style Conventions](https://doc.sagemath.org/html/en/developer/coding_basics.html).