Add some more guidance to issue template (#6692)

.github/ISSUE_TEMPLATE/issue_template.md

@@ -5,13 +5,18 @@ about: Report a bug or describe a new requested feature
 
 Click preview tab ^^^ above!
 
-By continuing to file this new issue / feature request, I confirm I have :
+By continuing to file this new issue / feature request, I confirm I have:
 1. searched the [live NEWS file](https://github.com/Rdatatable/data.table/blob/master/NEWS.md) to see if it has been fixed in dev already. If so, I tried the [latest dev version](https://github.com/Rdatatable/data.table/wiki/Installation#windows).
 2. looked at the titles of all the issues in the [current milestones](https://github.com/Rdatatable/data.table/milestones) and am aware of those. (Adding new information to existing issues is very helpful and appreciated.)
 3. [searched all issues](https://github.com/Rdatatable/data.table/issues) (i.e. not in a milestone yet) for similar issues to mine and will include links to them explaining why mine is different.
 4. searched on [Stack Overflow's data.table tag](http://stackoverflow.com/questions/tagged/data.table) and there is nothing similar there.
 5. read the [Support](https://github.com/Rdatatable/data.table/wiki/Support) and [Contributing](https://github.com/Rdatatable/data.table/blob/master/.github/CONTRIBUTING.md) guides.
-6. please don't tag your issue with text in the title; project members will add the appropriate tags later.
+
+Some general advice on the title and description fields for your PR
+
+- Please don't tag your issue with text in the title like '[Joins]'; project members will add the appropriate tags later.
+- Don't write text like 'Closes #xxx' in the PR title either; GitHub does not recognize this text, whereas GitHub auto-links issues in the description, [see docs](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword).
+- Title and Description fields should try and be self-contained as much as possible. The title answers "what is this change" and the description provides necessary details/thought processes/things tried but abandoned. Imagine visiting your PR in 5 years' time and trying to glean what it's all about quickly and without needing to open 10 new tabs.
 
 #### Thanks! Please remove the text above and include the two items below.