Link to enumeration of license abbreviations

[sbp]

The "licenses" field described in package.md requires the use of license abbreviations when provided. Linking to or including the full list of valid abbreviations would help package.json maintainers. I assume the abbreviations are the ones used on the OSI licenses list. Since the list would also have to be scraped and mirrored for use in the pre-commit package.json validator, perhaps a link to the list that the validator itself uses would be more appropriate, in case it becomes out of sync with the OSI version.

There was discussion on IRC on #jquery about this.

[scottgonzalez]

Looking into this more, I think we should not enforce this. We should link to the OSI licenses list and encourage specifying abbreviated names for the type, but I don't think we should require type. It's possible for someone to write a license that isn't named and use it for a plugin. The current requirement for licenses comes from CommonJS; npm doesn't even mention licenses in it's package.json spec.