add mon-fri to docs

Author: felixheck

README.md

@@ -147,7 +147,7 @@ MATCH_TIME=15:00
 
 ### `[MATCH_DAY=MON]`
 
-Pass in a single day in the `MON/TUE/WED/THU/FRI/SAT/SUN` format to define the day of announcing the matches.  
+Pass in a single day matching the `/^((MON-FRI)|MON|TUE|WED|THU|FRI|SAT|SUN)$/` pattern to define the day or range of days for announcing the matches.  
 This option affects both the Slack messages and the scheduled matches.
 
 Example: