Add vendor field, remove x-tool guidance

[nickelpro]

Fixes: #5

Unsure if it's desirable to allow this inside requirement as well. Conservatively I'm against it. If it's absolutely necessary we can add it later. Also on the fence about platform, but I think that allows CPS to be extended to environments the spec doesn't currently encompass.

[mwoehlke]

I am not convinced there is consensus on the name. See #5 (comment).

[mwoehlke]

Suggested change

	Vendors are *strongly* encouraged to record a ``version`` field in the
	|package| vendor object.
	Vendors are *strongly* encouraged to record a ``version`` field
	in the |package| vendor object.

However, it isn't clear what the intent is, here, and if you're essentially proposing that tools creating CPS files record their identity version... I'm not necessarily opposed (although if we're going to recommend that, I would much rather we do so as a supplemental schema), but I'm less convinces that should be done for the sake of "versioning" extensions. I would rather the extensions themselves be versioned so Tool A can potentially write extensions for Tool B. Maybe this is redundant, but the extension tag and namespace are already redundant any time an extension is used.

An example would also be beneficial. I actually wouldn't mind seeing a full documentation page on extensions.

[mwoehlke]

Suggested change

	Records vendor-specific extensions to CPS metadata. Vendors should ensure the
	namspace they record in the vendor object is consistent across all scopes.
	Records vendor-specific extensions to CPS metadata.
	Vendors should ensure that
	the namspace they record in the vendor object
	is consistent across all scopes.