github: update issue template

Update the issue template to point to the discussions repo.

Author: ethomson

.github/ISSUE_TEMPLATE

@@ -1,7 +1,12 @@
-You are opening a _bug report_ against the libgit2 project.  If you have a
-question about an API or usage, please ask on StackOverflow:
-http://stackoverflow.com/questions/tagged/libgit2.  Please fill out the
-reproduction steps (below) and delete this introductory paragraph.  Thanks!
+You are opening a _bug report_ against the libgit2 project: we use
+GitHub Issues for tracking bug reports and feature requests.  If you
+have a question about an API or usage, please ask on StackOverflow:
+http://stackoverflow.com/questions/tagged/libgit2.  If you want to
+have high-level discussions about the libgit2 project itself, visit
+https://github.com/libgit2/discussions.
+
+Otherwise, to report a bug, please fill out the reproduction steps
+(below) and delete these introductory paragraphs.  Thanks!
 
 ### Reproduction steps