Merge pull request #830 from cmonr/fix-cli-install-instructions

Updated mbed-cli install instructions to use 'pip' instead of 'setup.py'

Author: Amanda Butler

docs/tools/CLI/cli-setup.md/cli-reqs.md

@@ -104,7 +104,7 @@ $ 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 -e ./mbed-cli
 ```
 
 On Linux or macOS, you may need to run with `sudo`.