create: Show which flags are required in --help output

What does this MR do and why?

Fixes #96 (closed)

Before

NAME:
   help create - Create a Release using GitLab's Releases API https://docs.gitlab.com/ee/api/releases/#create-a-release

USAGE:
   help create [command options] [arguments...]

OPTIONS:
   --name value         The release name
   --description value  The description of the release; you can use Markdown. A file can be used to read the description contents, must exist inside the working directory; if it contains any whitespace, it will be treated as a string
   --tag-name value     The tag the release will be created from [$CI_COMMIT_TAG]
   --tag-message value  Message to use if creating a new annotated tag
   --ref value          If tag_name doesn’t exist, the release will be created from ref; it can be a commit SHA, another tag name, or a branch name [$CI_COMMIT_SHA]
   --assets-link value  JSON string representation of an asset link; (e.g. --assets-link='{"name": "Asset1", "url":"https://<domain>/some/location/1", "link_type": "other", "filepath": "xzy" }' or --assets-link='[{"name": "Asset1", "url":"https://example.com/some/location/1"}, {"name": "Asset2", "url":"https://example.com/some/location/2"}]'  (accepts multiple inputs)
   --milestone value    List of the titles of each milestone the release is associated with (e.g. --milestone "v1.0" --milestone "v1.0-rc)"; each milestone needs to exist                                                                                                                                                                               (accepts multiple inputs)
   --released-at value  The date when the release will be/was ready; defaults to the current time; expected in ISO 8601 format (2019-03-15T08:00:00Z)
   --help, -h           Show help (default: false)

After

NAME:
   help create - Create a Release using GitLab's Releases API https://docs.gitlab.com/ee/api/releases/#create-a-release

USAGE:
   help create [command options] [arguments...]

OPTIONS:
   --tag-name value     (required) The tag the release will be created from [$CI_COMMIT_TAG]
   --name value         The release name
   --description value  The description of the release; you can use Markdown. A file can be used to read the description contents, must exist inside the working directory; if it contains any whitespace, it will be treated as a string
   --tag-message value  Message to use if creating a new annotated tag
   --ref value          If tag_name doesn’t exist, the release will be created from ref; it can be a commit SHA, another tag name, or a branch name [$CI_COMMIT_SHA]
   --assets-link value  JSON string representation of an asset link; (e.g. --assets-link='{"name": "Asset1", "url":"https://<domain>/some/location/1", "link_type": "other", "filepath": "xzy" }' or --assets-link='[{"name": "Asset1", "url":"https://example.com/some/location/1"}, {"name": "Asset2", "url":"https://example.com/some/location/2"}]'  (accepts multiple inputs)
   --milestone value    List of the titles of each milestone the release is associated with (e.g. --milestone "v1.0" --milestone "v1.0-rc)"; each milestone needs to exist                                                                                                                                                                               (accepts multiple inputs)
   --released-at value  The date when the release will be/was ready; defaults to the current time; expected in ISO 8601 format (2019-03-15T08:00:00Z)
   --help, -h           Show help (default: false)

Checklist

• I added tests
• Green pipeline
• Assign to reviewer