Very short description of exit codes and how to access them

[Rowlando13]

No description provided.

[Rowlando13]

@kdeldycke Can you sanity check this. I wrote it based on internet research, but I have not used explicitly used exit codes.

[Rowlando13]

The context is that I was documenting #2909 and some things about command and group exit codes. The long term idea is that new comers to Click don't need to be experts on the command line to understand our docs, so I am planning to include some very brief primers in key areas such as #3076

[kdeldycke]

Yeah I'm OK with this. I was also thinking about #2909 . It is worth merging your PR if you can manage to explain the different exit code between having explicitly requested --help, and the help screen showing up as a invite to the user to fix its CLI invocation.