Added documents

docs/zh/Assemble-context-zh.md

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+# 组合上下文API
+
+```kotlin
+public val CustomProtocolContext: ProtocolContextBuilder.() -> Unit = {
+    rsaEncryptor = RSA1024Encryptor { key, data -> ... }
+    sha1Hasher = Sha1Hasher { serverId, secretKey, publicKey -> ... }
+    cipherFactory = { key -> ... }
+    authProvider = AuthenticationProvider { url, accessToken, uuid, serverIdHash -> ... }
+    socketEngine = MyCustomTCPSocketImpl()
+}
+```
+
+> 如果使用`libmc-protocol-context`中的`DefaultProtocolContext`作为默认上下文实现,
+> 请参阅[实现 TCPSocket](Impl-TCP-Socket-zh.md)
+
+# 修改默认实现
+
+```kotlin
+fun main() {
+    val myNewProtocolContext = DefaultProtocolContext.withCustom {
+        rsaEncryptor = RSA1024Encryptor { key, data -> ... }
+        sha1Hasher = Sha1Hasher { serverId, secretKey, publicKey -> ... }
+        cipherFactory = { key -> ... }
+        authProvider = AuthenticationProvider { url, accessToken, uuid, serverIdHash -> ... }
+        socketEngine = MyCustomTCPSocketImpl()
+    }
+}
+```
+
+> 使用`.withCustom`来修改已有的上下文API实现
+