mention the examples in the readme

Author: dgoffredo

README.md

@@ -35,6 +35,7 @@ int main() {
     std::this_thread::sleep_for(std::chrono::seconds(2));
 }
 ```
+See the [examples](examples) directory for more extensive usage examples.
 
 Build
 -----

examples/README.md

@@ -0,0 +1,10 @@
+Example Usage
+=============
+This directory contains two example projects that illustrate how dd-trace-cpp
+can be used to add Datadog tracing to a C++ application.
+
+- [hasher](hasher) is a command-line tool that creates a complete trace
+  involving only one service.
+- [http-server](http-server) is an ensemble of services, including one C++
+  service traced using this library. The traces generated are distributed
+  across all of the services in the example.