Link Go agent project in main repo. (#2697)

* Link Go agent project in main repo.

* Fix link.

README.md

@@ -34,7 +34,8 @@ The core features are following.
 SkyWalking supports to collect telemetry (traces and metrics) data from multiple sources
 and multiple formats,
 including
-1. Java, .NET Core, NodeJS and PHP auto-instrument agents in SkyWalking format
+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. Zipkin v1/v2 format
 1. Jaeger gRPC format.

docs/en/setup/README.md

@@ -17,6 +17,7 @@ You could go to their project repositories to find out the releases and how to u
 - [SkyAPM .NET Core agent](https://github.com/SkyAPM/SkyAPM-dotnet). See .NET Core agent project document for more details.
 - [SkyAPM Node.js agent](https://github.com/SkyAPM/SkyAPM-nodejs). See Node.js server side agent project document for more details.
 - [SkyAPM PHP SDK](https://github.com/SkyAPM/SkyAPM-php-sdk). See PHP agent project document for more details.
+- [Tetrate GO2Sky](https://github.com/tetratelabs/go2sky). See GO2Sky project document for more details.
 
 ## Service Mesh
   - Istio