src/xbps/index.md: Link to void-packages/Manual.md

[camoz]

Add link to instructions for creating new packages.

[camoz]

I now ran vmdfmt over the changes.

[ericonr]

See Contributing for a general overview of how to contribute and the Manual for details of how to create source packages.

The README has this as its third paragraph, does it even make sense to mention here?

[camoz]

@ericonr IMO it would still be helpful to have a reference about creating templates from here, since this is probably the place where users start looking/searching. If we don't link to the Manual from here, then I would at least mention that information about creating templates can be (indirectly) found in the Readme (even if it's just another link there). But somewhere there should be a reference here about how to create templates IMO.