Add wasm plugin contribution introduction (#47)

plugins/wasm-go/extensions/hello-world/main.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2022 Alibaba Group Holding Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"github.com/tetratelabs/proxy-wasm-go-sdk/proxywasm"
+	"github.com/tetratelabs/proxy-wasm-go-sdk/proxywasm/types"
+
+	"github.com/alibaba/higress/plugins/wasm-go/pkg/wrapper"
+)
+
+func main() {
+	wrapper.SetCtx(
+		"hello-world",
+		wrapper.ProcessRequestHeadersBy(onHttpRequestHeaders),
+	)
+}
+
+type HelloWorldConfig struct {
+}
+
+func onHttpRequestHeaders(ctx *wrapper.CommonHttpCtx[HelloWorldConfig], config HelloWorldConfig, needBody *bool, log wrapper.LogWrapper) types.Action {
+	err := proxywasm.AddHttpRequestHeader("hello", "world")
+	if err != nil {
+		log.Critical("failed to set request header")
+	}
+	proxywasm.SendHttpResponse(200, nil, []byte("hello world"), -1)
+	return types.ActionContinue
+}