Update usage.

README.md

@@ -27,8 +27,8 @@ Usage of rtlamr:
   -cpuprofile=: write cpu profile to this file
   -duration=0: time to run for, 0 for infinite, ex. 1h5m10s
   -fastmag=false: use faster alpha max + beta min magnitude approximation
-  -filterid=0: display only messages matching given id
-  -filtertype=0: display only messages matching given type
+  -filterid=: display only messages matching an id in a comma-separated list of ids.
+  -filtertype=: display only messages matching a type in a comma-separated list of types.
   -format=plain: format to write log messages in: plain, csv, json, xml or gob
   -gobunsafe=false: allow gob output to stdout
   -logfile=/dev/stdout: log statement dump file