docs: Update install to use helics[cli] (#69)

docs/installation.md

@@ -15,12 +15,14 @@ $ which pip    # sanity check on Unix
 $ where python # sanity check on Windows
 $ where pip    # sanity check on Windows
 
-$ python -m pip install helics
+$ python -m pip install 'helics[cli]'
 ```
 
 Using `python -m pip` invokes the `pip` module from the `python` process. This is the safest way to ensure you are installing `helics` into the place `python` will look for packages.
 If `pip` and `python` belong to the same environment, you can invoke `pip` directly.
 
+It is recommended to use the optional `[cli]` extension on the PyHELICS install to provide the use of the "runner" functionality for launching co-simulations (among other features) All of the [HELICS User Guide examples](https://docs.helics.org/en/latest/user-guide/examples/examples_index.html) use the runner.
+
 This will give you the latest version of the python helics interface.
 If you already have helics installed, you can upgrade to the latest version by using the following: