6.x.doc (#1627)

* update build md

* add   generated-sources  protobuf  in  build.md

docs/en/guides/How-to-build.md

@@ -25,6 +25,7 @@ This document helps people to compile and build the project in your maven and se
 1. Set **Generated Source Codes** folders.
     * `grpc-java` and `java` folders in **apm-protocol/apm-network/target/generated-sources/protobuf**
     * `grpc-java` and `java` folders in **oap-server/server-core/target/generated-sources/protobuf**
+    * `grpc-java` and `java` folders in **oap-server/server-receiver-plugin/skywalking-istio-telemetry-receiver-plugin/target/generated-sources/protobuf**
     
 ## Setup your Eclipse IDE
 1. Import the project as a maven project