update README.md

Author: atwayne

README.md

@@ -1,2 +1,39 @@
 # TestServer
-Yet another mock http server
+
+`TestServer` is designed for unit test. It generates an `HttpClient` instance that you can inject to your service.
+
+## Get Started
+
+1. Install the package by nuget
+    ```bash
+    dotnet add package Dubstep.TestUtilities.TestServer
+    ```
+2. Create a `TestServer` instance by `new TestServer()`
+3. Configure `RuleSet` of the `TestServer` instance
+4. Generate a HttpClient by `CreateClient()`
+
+## Example
+```csharp
+public async Task ShouldGetResponse {
+    // Arrange
+    var server = new TestServer();
+    var urlPattern = "\\?id=1";
+    var okResponse = "[1,2,3]";
+    server.CurrentRuleSet
+        .AddRule()
+        .WhenGet()
+        .WhenUrlMatch(urlPattern)
+        .SetOkResponse(okResponse);
+    var client = server.CreateClient();
+
+    // Act
+    var response = await client.GetAsync("/?id=1");
+    
+    // Assert
+    response.EnsureSuccessStatusCode();
+    var message = await response.Content.ReadAsStringAsync();
+    Assert.AreEqual(okResponse, message);
+}
+```
+
+Check [the unit test cases](https://github.com/atwayne/TestServer/blob/master/test/TestServer.Tests/TestServerTests.cs) for more examples