Update official documentation link. (#11)

diff --git a/README.md b/README.md
index b582111..12d303f 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@
 
 ## Documentation
 
-* [Official documentation](docs/README.md)
+* [Official documentation](https://skywalking.apache.org/docs/#PHPAgent)
 
 ## Installation Requirements