document regex match counting in the README

Describes -RPc and -CPc alongside the regex matching options they
modify. Kept with the feature rather than on the release branch, so the
README there documents only what that branch actually provides.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -88,7 +88,8 @@ The most commonly useful options (`wikiq --help` describes them all):
 - `-RP REGEX -RPl LABEL` searches revision text for a regular expression
   and reports the matches in a column named by the label; repeat the
   pair for multiple patterns. `-CP`/`-CPl` do the same for edit
-  summaries.
+  summaries. Adding `-RPc` or `-CPc` reports the number of matches
+  instead of the matched text.
 - `--resume` continues an interrupted run from the last complete line of
   an existing JSONL output file. Combined with `--time-limit HOURS`,
   this supports processing very large dumps in bounded chunks.