Add warning when hiredis not installed. Recommend installation. (#1621)

Author: adiamzn

redis/connection.py

@@ -71,6 +71,9 @@
     # only use byte buffer if hiredis supports it
     if not HIREDIS_SUPPORTS_BYTE_BUFFER:
         HIREDIS_USE_BYTE_BUFFER = False
+else:
+    msg = "redis-py works best with hiredis. Please consider installing"
+    warnings.warn(msg)
 
 SYM_STAR = b'*'
 SYM_DOLLAR = b'$'