crashes: readme: add reminder to add Fixes #abcde to prs to automatic…

…ally close issues.

tests/crashes/README.md

@@ -14,3 +14,10 @@ If you happen to fix one of the crashes, please move it to a fitting
 subdirectory in `tests/ui` and give it a meaningful name.
 Also please add a doc comment at the top of the file explaining why
 this test exists. :)
+Adding
+Fixes #NNNNN
+Fixes #MMMMM
+to the description of your pull request will ensure the
+corresponding tickets will be closed automatically upon merge.
+The ticket ids can be found in the file name or the `known-bug` annotation
+inside the testfile.