Update README.md (protocolbuffers#7329)

Author: anandolee

python/README.md

@@ -22,10 +22,8 @@ package.
 Development Warning
 ===================
 
-The Python implementation of Protocol Buffers is not as mature as the C++
-and Java implementations.  It may be more buggy, and it is known to be
-pretty slow at this time.  If you would like to help fix these issues,
-join the Protocol Buffers discussion list and let us know!
+The pure python performance is slow. For better preformance please
+use python c++ implementation.
 
 Installation
 ============