added basic readme to espnow lib

client/client/lib/espnow/README

+# basic usage
+
+To send data using espnow, create a new Message object,
+then use the add_data(value, identifier) method for every value
+to fill the message.
+when every value is added, use the send() method to send the data
+to the host (fipy). If the esp client has never recieved a config
+message from the host, it will instead broadcast the message.
+
+---
+right now, it is not possible to add more than 10 values.