Improve command documentation

[interstateone]

It was previously not clear how to specify versions of Xcode to install. The current version argument parsing is sort of incidentally a little fuzzy, and so there are sometimes different ways to specify a single pre-release version like "11.2.1 GM Seed", such as "11.2.1GM Seed". This change adds examples to the command help and to the project README in order to give examples of install commands for pre-release versions. I don't think this is totally sufficient and there is more to be done, but it's a first step.

Resolves #80

[landonepps]

Thanks, that really clears things up!
I had no idea you could specify a local xip file.

[MattKiazyk]

👍

Another enhancement would be to print out those examples if one command fails! So if it fails with the install command - print the install help.

mattkiazyk@Matts-MBP-2 xcodes % xcodes --help     
Usage:
  xcodes
  xcodes [command]

Available Commands:
  install    Download and install a specific version of Xcode
  installed  List the versions of Xcode that are installed
  list       List all versions of Xcode that are available to install
  uninstall  Uninstall a specific version of Xcode
  update     Update the list of available versions of Xcode
  version    Print the version number of xcodes itself

Use "xcodes [command] --help" for more information about a command.
mattkiazyk@Matts-MBP-2 xcodes % xcodes install --help
Download and install a specific version of Xcode

Usage:
  xcodes install <version> [flags]

Examples:
xcodes install 10.2.1
xcodes install 11 Beta 7
xcodes install 11.2 GM seed
xcodes install 9.0 --url ~/Archive/Xcode_9.xip

Flags:
      --url string  Local path to Xcode .xip 

[MattKiazyk]

Works as intended 👍

Suggestion was added in comment for future enhancement