add helm hub china in README.md (helm#15004)

Signed-off-by: 天元 <jianbo.sjb@alibaba-inc.com>

README.md

@@ -99,3 +99,7 @@ To provide that support the API versions of objects should be those that work fo
 ## Status of the Project
 
 This project is still under active development, so you might run into [issues](https://github.com/helm/charts/issues). If you do, please don't be shy about letting us know, or better yet, contribute a fix or feature.
+
+## Happy helming in China
+
+If you are in China, there are some problems to use Helm Charts directly (e.g. gcr.io, googleapis.com etc), you can use this mirror repo at https://github.com/cloudnativeapp/charts.