readme: add a note about identify() return value

The call returns a string value instead of an object of type UUID. It
is not obvious. The patch adds a note about it into README.md.

Closes #182

Author: oleg-jukovec

README.md

@@ -479,6 +479,10 @@ with the UUID of the session.
 In case of attempt to use an invalid format UUID or expired UUID, an error will
 be thrown.
 
+Be careful, UUID here is a 16-bit string generated by
+[uuid.bin()](https://www.tarantool.io/en/doc/latest/reference/reference_lua/uuid/#lua-function.uuid.bin),
+not an object of type UUID.
+
 Usage example:  
 Sometimes we need an ability to acknowledge a task after reconnect (because
 retrying it is undesirable) or even acknowlegde using another connection.