Add documentation on how to run examples

Author: Katia Bazzi

README.md

@@ -18,6 +18,13 @@ Please see the [example programs](examples/) for examples of how to use this lib
 * [Trivial Example](examples/trivial/main.py) shows how to use the library on a single host.
 * [Context in Headers](examples/http/context_in_headers.py) shows how to pass a `TraceContext` through `HTTP` headers.
 
+You can run the examples by doing:
+```python
+tox
+source .tox/py27/bin/activate
+python examples/nontrivial/main.py
+```
+
 Or if your python code is already instrumented for OpenTracing, you can simply switch to LightStep's implementation with:
 
 ```python