Document supported pattern syntax in README

Add a Pattern syntax section listing the five pattern forms accepted
by qBittorrent's excluded file names (suffix, prefix, contains, exact,
regex), which are the same forms that the entries in blacklist and
whitelist must use. Note that the merge script subtracts whitelist
from blacklist as exact strings regardless of pattern form, so
equivalent patterns written differently are not interchangeable.

README.md

@@ -123,6 +123,29 @@ from upstream. Removals must go through the whitelist.
 See [Lists](https://git.hisp.no/arr/blocklists/wiki/Lists) for the full
 rules and worked examples.
 
+## Pattern syntax
+
+Each line in `blacklist` and `whitelist` is a pattern in one of the
+following forms (the same forms that qBittorrent's excluded file names
+accepts, since that is where blacklist patterns end up via Cleanuparr's
+Blocklist Sync):
+
+| Form | Example | Matches |
+|---|---|---|
+| `*example` | `*.srt` | File name ends with `example` |
+| `example*` | `sample*` | File name starts with `example` |
+| `*example*` | `*sample*` | File name contains `example` |
+| `example` | `RARBG.txt` | File name is exactly `example` |
+| `regex:<regex>` | `regex:.*\.srt$` | File name matches the regex |
+
+The merge script subtracts the whitelist from the blacklist as **exact
+strings**, not by pattern semantics. To remove a pattern from the
+blacklist, the whitelist line must be byte-for-byte identical to the
+blacklist line. `*.srt` in the whitelist strips `*.srt` from the
+blacklist; it does **not** strip `regex:.*\.srt$` even though both
+patterns describe the same files. See
+[Lists](https://git.hisp.no/arr/blocklists/wiki/Lists) for the full rules.
+
 ## Sync schedule
 
 The Gitea Actions workflow runs every 7 days at 04:00 UTC and on manual