add swagger demo

cmd/open_im_api/docs/swagger.yaml

@@ -0,0 +1,42 @@
+basePath: /api/v1
+host: localhost:8080
+info:
+  contact:
+    email: support@swagger.io
+    name: API Support
+    url: http://www.swagger.io/support
+  description: This is a sample server celler server.
+  license:
+    name: Apache 2.0
+    url: http://www.apache.org/licenses/LICENSE-2.0.html
+  termsOfService: http://swagger.io/terms/
+  title: Swagger Example API
+  version: "1.0"
+paths:
+  /hello:
+    get:
+      consumes:
+      - application/json
+      description: 向你说Hello
+      parameters:
+      - description: 人名
+        in: query
+        name: who
+        required: true
+        type: string
+      responses:
+        "200":
+          description: '{"msg": "hello Razeen"}'
+          schema:
+            type: string
+        "400":
+          description: '{"msg": "who are you"}'
+          schema:
+            type: string
+      summary: 测试SayHello
+      tags:
+      - 测试
+securityDefinitions:
+  BasicAuth:
+    type: basic
+swagger: "2.0"