Add note about using expirationd with replication

Closes #14

README.md

@@ -28,7 +28,16 @@ Table below may help you to choose a proper module for your requirements:
 
 ### Documentation
 
-See https://tarantool.github.io/expirationd/
+See API documentation in https://tarantool.github.io/expirationd/
+
+Note about using expirationd with replication: by default expirationd should
+process tasks only on the writeable instance, it means that expirationd *will
+not* start task processing on a replica. Here the word 'replica' means an
+instance with at least one configured upstream, it's an option
+`box.cfg.replication_source` (`box.cfg.replication` for Tarantool 1.7.6+). One
+can force running task on replica with option `force` in `start()` module
+function. The option force let a user control where to start task processing
+and where don't.
 
 ### Examples