Clarify that RDFa and JSON-LD are supported formats

The getting started documentation referred to a long-term plan to
support a broader variety of formats, but RDFa and JSON-LD are
pretty tightly integrated into the rest of the schema.org documentation,
so we might as well mention them explicitly in the getting started (but
continue documenting with microdata as the default approach).

Signed-off-by: Dan Scott <dan@coffeecode.net>