CF-CLI installation guide reference

[europ]

Description:

• added reference to the cf CLI installation guide

[nibalizer]

This looks good, but we could also go further and encourage people to download ibmcloud and use ibmcloud app

[europ]

you should use ibmcloud cf instead of ibmcloud app

$ ibmcloud help app
NAME:
   ibmcloud app - Manage Cloud Foundry applications and application related domains and routes. (Deprecated, use 'ibmcloud cf' commands.)
...