Write up reporting.

docs/Using-Mastodon/User-guide.md

@@ -182,4 +182,8 @@ The blocked user will not be notified of your blocking them. They will be remove
 
 #### Reporting Toots or Users
 
-[TODO]
+If you encounter a toot or a user that is breaking the rules of your instance or that you otherwise want to draw the instance administrators' attention to (e.g. if someone is harassing another user, spamming pornography or posting illegal content), you can click the "..." menu button on the toot or the "hamburger" menu on the profile and select to report this. The rightmost column will then switch over to the following form:
+
+![Report form](screenshots/report.png)
+
+In this form, you can select any toots you would like to report to the instance administrators and fill in any comment that might be helpful in identifying or handling the issue (from "is a spammer" to "this post contains untagged pornography"). The report will be visible to server administrators once it is sent so they can take appropriate action, for example hiding the user's posts from the public timeline or banning their account.