Add self-training material for writing a good bug report.

Some existing bug reports are not easy to read. Add an external link to show users what a high-quality bug report looks like.

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -10,6 +10,15 @@ body:
     options:
     - label: I have read and agree to adhere to the [Code of Conduct](https://github.com/oxen-io/session-ios/blob/master/CODE_OF_CONDUCT.md)
       required: true
+
+- type: checkboxes
+  attributes:
+    label: Self-training on how to write a bug report
+    description: High quality bug report can help the team save time and improve the chance of getting fixed. Please read [how to write a bug report](https://www.browserstack.com/guide/how-to-write-a-bug-report) before submitting your issue.
+    options:
+    - label: I have learned [how to write a bug report](https://www.browserstack.com/guide/how-to-write-a-bug-report)
+      required: true
+
 - type: checkboxes
   attributes:
     label: Is there an existing issue for this?