Officially add LUA agent link in the main repo and document. (#4402)

README.md

@@ -35,9 +35,10 @@ SkyWalking supports to collect telemetry (traces and metrics) data from multiple
 and multiple formats,
 including
 1. Java, [.NET Core](https://github.com/SkyAPM/SkyAPM-dotnet), [NodeJS](https://github.com/SkyAPM/SkyAPM-nodejs) and [PHP](https://github.com/SkyAPM/SkyAPM-php-sdk) auto-instrument agents in SkyWalking format
-1. Manual-instrument [Go agent](https://github.com/tetratelabs/go2sky) in SkyWalking format.
-1. Istio telemetry format
-1. Envoy gRPC Access Log Service (ALS) format in Istio controlled service mesh
+1. [Go agent](https://github.com/tetratelabs/go2sky).
+1. [LUA agent](https://github.com/apache/skywalking-nginx-lua), especially for Nginx, OpenResty.
+1. Envoy gRPC Access Log Service (ALS) format in Istio controlled service mesh. 
+1. Istio telemetry format.
 1. Envoy Metrics Service format.
 1. Zipkin v1/v2 format.
 1. Jaeger gRPC format.

docs/en/setup/README.md

@@ -14,7 +14,9 @@ If you have any issues, please check that your issue is not already described in
 
 - [Java agent](service-agent/java-agent/README.md). Introduces how to install java agent to your service, without any impact in your code.
 
-The following agents and SDKs are compatible with the SkyWalking's samples formats and protocols, but are maintained by 3rd-parties.
+- [LUA agent](https://github.com/apache/skywalking-nginx-lua). Introduce how to install the lua agent in Nginx + LUA module or OpenResty.
+
+The following agents and SDKs are compatible with the SkyWalking's data formats and network protocols, but are maintained by 3rd-parties.
 You can go to their project repositories for additional info about guides and releases.
 
 - [SkyAPM .NET Core agent](https://github.com/SkyAPM/SkyAPM-dotnet). See .NET Core agent project document for more details.