mention cf CLI exit codes

[dkoper]

We should mention that the cf CLI uses exit codes, useful for scripting: '0' means command was successful, any other value means it failed.
A new section in "Consistent Behavior to Help You Work Efficiently" in installcf/whats-new-v6.html.md.erb may be a good location.
We could include the commands used to check the exit code on each supported platform (Linux/Mac, Windows Command Line, Windows Powershell), as done here http://docs.aws.amazon.com/cli/latest/topic/return-codes.html.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. You can view the current status of your issue at: https://www.pivotaltracker.com/story/show/110476630.

[abbyachau]

Hi @dkoper, many thanks for confirming we can close this issue since it is covered in the larger rewrite of the cf CLI docs (see: #58).

Best,

Abby