添加readme.md文件

README.MD

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+ASP.NET CORE中间件相关扩展。
+
+中间件基类`MiddlewareBase`
+
+可以自行继承并实现自己的中间件类。例如：  
+```
+    /// <summary>
+    /// 测试中间件
+    /// </summary>
+    public class MiddlewareTest:MiddlewareBase
+    {
+        public MiddlewareTest(RequestDelegate next) : base(next) {
+        }
+
+        /// <summary>
+        /// 在调用next之后执行
+        /// </summary>
+        /// <param name="context">请求上下文</param>
+        public async override void InvokeNextAfter(HttpContext context) {
+            await context.Response.WriteAsync("\nInvokeNextAfter");
+        }
+        /// <summary>
+        /// 在调用next之前执行
+        /// </summary>
+        /// <param name="context">请求上下文</param>
+        public async override void InvokeNextBefore(HttpContext context) {
+            await context.Response.WriteAsync("InvokeNextBefore\n");
+        }
+    }
+
+```   
+然后修改setup.cs
+```
+public void Configure(IApplicationBuilder app,IWebHostEnvironment env) {
+  if(env.IsDevelopment()) {
+    app.UseDeveloperExceptionPage();
+  }
+
+  app.UseRouting();
+
+  app.UseMiddleware<MiddlewareTest>();
+
+  app.UseEndpoints(endpoints => {
+    endpoints.MapGet("/",async context => {
+      await context.Response.WriteAsync("Hello World!");
+    });
+  });
+}
+```  
+浏览器发送请求后：   
+```
+InvokeNextBefore
+Hello World!
+InvokeNextAfter
+```