Clarify generated README for bundled packages

[danielbachhuber]

For packages bundled in the Phar (https://github.com/wp-cli/scaffold-command), we should make sure the INSTALLING and CONTRIBUTING sections are clarified appropriately.

Installing

• Mention that the package is bundled with WP-CLI

Contributing

• Update instructions to reflect package being bundled with WP-CLI

[danielbachhuber]

Updating contributing instructions is blocked by wp-cli/handbook#17