Add information on the different ways to specify a package in to the help section.

[dgnsrekt]

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

Add the different ways a package can be added to the poetry add --help DESCRIPTION section. This information only shows up on poetry init. I think it would be useful as a quick reference when adding future dependencies with the poetry add command.

Example

➜ poetry add --help
USAGE
  poetry add [-D] [-E <...>] [--optional] [--python <...>] [--platform <...>] [--allow-prereleases] [--dry-run] <name1> ... [<nameN>]

ARGUMENTS
  <name>                 The packages to add.

OPTIONS
  -D (--dev)             Add as a development dependency.
  -E (--extras)          Extras to activate for the dependency. (multiple values allowed)
  --optional             Add as an optional dependency.
  --python               Python version for which the dependency must be installed.
  --platform             Platforms for which the dependency must be installed.
  --allow-prereleases    Accept prereleases.
  --dry-run              Output the operations but do not execute anything (implicitly enables --verbose).

GLOBAL OPTIONS
  -h (--help)            Display this help message
  -q (--quiet)           Do not output any message
  -v (--verbose)         Increase the verbosity of messages: "-v" for normal output, "-vv" for more verbose output and "-vvv" for debug
  -V (--version)         Display this application version
  --ansi                 Force ANSI output
  --no-ansi              Disable ANSI output
  -n (--no-interaction)  Do not ask any interactive question

DESCRIPTION
  The add command adds required packages to your pyproject.toml and installs them.
  
  If you do not specify a version constraint, poetry will choose a suitable one based on the available package versions.

  You can specify a package in the following forms:
    - A single name (requests)
    - A name and a constraint (requests ^2.23.0)
    - A git url (git+https://github.com/python-poetry/poetry.git)
    - A git url with a revision (git+https://github.com/python-poetry/poetry.git#develop)
    - A file path (../my-package/my-package.whl)
    - A directory (../my-package/)
    - A url (https://example.com/packages/my-package-0.1.0.tar.gz)

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