Explain how subdomains work in README

README.md

@@ -26,12 +26,18 @@ $list = file('spammers.txt', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
 
 ## Contributing
 
-To add a new referrer spammer to the list, [click here to edit the spammers.txt file](https://github.com/piwik/referrer-spam-blacklist/edit/master/spammers.txt) and create a pull request. Alternatively you can create a [new issue](https://github.com/piwik/referrer-spam-blacklist/issues/new). In you issue or pull request please **explain where the referrer domain appeared and why you think it is a spammer**.
+To add a new referrer spammer to the list, [click here to edit the spammers.txt file](https://github.com/piwik/referrer-spam-blacklist/edit/master/spammers.txt) and create a pull request. Alternatively you can create a [new issue](https://github.com/piwik/referrer-spam-blacklist/issues/new). In your issue or pull request please **explain where the referrer domain appeared and why you think it is a spammer**.
 
 If you open a pull request, it is appreciated if you keep one hostname per line, keep the list ordered alphabetically, and use [Linux line endings](http://en.wikipedia.org/wiki/Newline).
 
 Please [search](https://github.com/piwik/referrer-spam-blacklist/issues) if somebody already reported the host before opening a new one.
 
+### Subdomains
+
+Piwik does sub-string matching on domain names from this list, so adding `semalt.com` is enough to block all subdomain referrers too, such as `semalt.semalt.com`.
+
+However, there are cases where you'd only want to add a subdomain but not the root domain. For example, add `referrerspammer.tumblr.com` but not `tumblr.com`, otherwise all `*.tumblr.com` sites would be affected.
+
 ### Sorting
 
 To keep the list sorted the same way across forks it is recommended to let the computer do the sorting. The list follows the merge sort algorithm as implemented in [sort](https://en.wikipedia.org/wiki/Sort_(Unix)). You can use sort to both sort the list and filter out doubles: