document how to quote a comment in a reply (gitlab#34)

tsa/howto/gitlab.md

@@ -68,6 +68,17 @@ are similar.
 If you do not have access to GitLab, please use one of the mailing
 lists: <tor-dev@lists.torproject.org> would be best.
 
+## How to quote a comment in a reply?
+
+The "Reply" button only creates a new comment without any quoted text
+by default.  It seems the solution to that is currently highlighting
+the text to quote and then pressing the `r`-key. See also the [other
+keyboard shortcuts](https://docs.gitlab.com/ee/user/shortcuts.html).
+
+Alternatively, you can copy-paste the text in question in the comment
+form, select the pasted text, and hit the `Insert a quote` button
+which look like a styled, curly, and closing quotation mark `”`.
+
 # How-to
 
 ## Hooking up a project with the bots