Replaced instance of 'python setup.py' with 'pip install'

Author: Cruz Monrreal

README.md

@@ -105,7 +105,7 @@ $ git clone https://github.com/ARMmbed/mbed-cli
 Once cloned, you can install Mbed CLI as a Python package:
 
 ```
-$ python setup.py install
+$ pip install .
 ```
 
 On Linux or Mac, you may need to run with `sudo`.