add doc for TraceCrossThread (#1067)

* add doc for TraceCrossThread

docs/README.md

@@ -21,6 +21,7 @@
       * [log4j2](en/Application-toolkit-log4j-2.x.md)
       * [logback](en/Application-toolkit-logback-1.x.md)
     * [Trace](en/Application-toolkit-trace.md)
+    * [Propagate Context across Thread](en/Application-toolkit-trace-cross-thread.md)
   * Testing
     * [Plugin Test](https://github.com/SkywalkingTest/agent-integration-test-report)
     * [Java Agent Performance Test](https://skywalkingtest.github.io/Agent-Benchmarks/)

docs/README_ZH.md

@@ -21,6 +21,7 @@
       * [log4j2组件](cn/Application-toolkit-log4j-2.x-CN.md)
       * [logback组件](cn/Application-toolkit-logback-1.x-CN.md)
     * [Trace](cn/Application-toolkit-trace-CN.md)
+    * [调用链跨线程传递](cn/Application-toolkit-trace-cross-thread-CN.md) 
   * 测试用例
     * [插件测试](https://github.com/SkywalkingTest/agent-integration-test-report)
     * [Java 探针性能测试](https://skywalkingtest.github.io/Agent-Benchmarks/README_zh.html)
@@ -38,4 +39,4 @@
   * FAQ
     * [Trace查询有数据，但是没有拓扑图和JVM数据?](cn/FAQ/Why-have-traces-no-others-CN.md)
     * [加载探针，Console被GRPC日志刷屏](cn/FAQ/Too-many-gRPC-logs-CN.md)
-    * [Kafka消息消费端链路断裂](cn/FAQ/Kafka-plugin-CN.md)
\ No newline at end of file
+    * [Kafka消息消费端链路断裂](cn/FAQ/Kafka-plugin-CN.md)

docs/cn/Application-toolkit-trace-cross-thread-CN.md

+# 跨线程追踪
+* 使用 maven 和 gradle 依赖相应的工具包
+```xml
+   <dependency>
+      <groupId>org.apache.skywalking</groupId>
+      <artifactId>apm-toolkit-trace</artifactId>
+      <version>${skywalking.version}</version>
+   </dependency>
+```
+
+* 使用方式一.
+```java
+    @TraceCrossThread
+    public static class MyCallable<String> implements Callable<String> {
+        @Override
+        public String call() throws Exception {
+            return null;
+        }
+    }
+...
+    ExecutorService executorService = Executors.newFixedThreadPool(1);
+    executorService.submit(new MyCallable());
+```
+* 使用方式二.
+```java
+    ExecutorService executorService = Executors.newFixedThreadPool(1);
+    executorService.submit(CallableWrapper.of(new Callable<String>() {
+        @Override public String call() throws Exception {
+            return null;
+        }
+    }));
+```
+_示例代码，仅供参考_
+
+
+
+
+

docs/en/Application-toolkit-trace-cross-thread.md

+# trace cross thread
+* Dependency the toolkit, such as using maven or gradle
+```xml
+   <dependency>
+      <groupId>org.apache.skywalking</groupId>
+      <artifactId>apm-toolkit-trace</artifactId>
+      <version>${skywalking.version}</version>
+   </dependency>
+```
+
+* usage 1.
+```java
+    @TraceCrossThread
+    public static class MyCallable<String> implements Callable<String> {
+        @Override
+        public String call() throws Exception {
+            return null;
+        }
+    }
+...
+    ExecutorService executorService = Executors.newFixedThreadPool(1);
+    executorService.submit(new MyCallable());
+```
+* usage 2.
+```java
+    ExecutorService executorService = Executors.newFixedThreadPool(1);
+    executorService.submit(CallableWrapper.of(new Callable<String>() {
+        @Override public String call() throws Exception {
+            return null;
+        }
+    }));
+```
+_Sample codes only_
+
+
+