issue template: put instructions in comments

People don't always think to remove the instructions when filing an issue. Putting them
in markdown comment blocks at least prevents
them from visually polluting the resulting issue.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -14,7 +14,7 @@ Knitting machine:
 AYAB hardware: 
 
 **Describe the bug**
-A clear and concise description of what the bug is.
+<!-- A clear and concise description of what the bug is. -->
 
 **To Reproduce**
 Steps to reproduce the behavior:
@@ -24,10 +24,10 @@ Steps to reproduce the behavior:
 4. See error
 
 **Expected behavior**
-A clear and concise description of what you expected to happen.
+<!-- A clear and concise description of what you expected to happen. -->
 
 **Screenshots**
-If applicable, add screenshots to help explain your problem.
+<!-- If applicable, add screenshots to help explain your problem. -->
 
 **Additional context**
-Add any other context about the problem here.
+<!-- Add any other context about the problem here. -->