Update PR template.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

Author: sjaeckel

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,7 +2,11 @@
 
 Thank you for your pull request.
 
-If this fixes an existing github issue, make sure to have a line saying 'Fixes #XXXX' (without quotes) in the commit message.
+If this fixes an existing github issue, make sure to have a line saying 'Fixes: <github-issue>' (without quotes) in the commit message.
+Please use the long format to refer to an issue 'libtom/libtomcrypt#<issue-id>'.
+
+If this fixes something (even if there exists no github issue), make sure to have a line saying 'Fixes: <commit-id> ("Description")'
+This can be created via e.g. `git --no-pager log -1 --pretty=fixes <commid-id>
 
 -->
 
@@ -11,3 +15,4 @@ If this fixes an existing github issue, make sure to have a line saying 'Fixes #
 
 * [ ] documentation is added or updated
 * [ ] tests are added or updated
+* [ ] if this fixes something: added a `Fixes:` tag to the commit message