changed section on RDF + added JSON-LD info

[elf-pavlik]

i didn't see reference to JSON-LD so thought about adding it here...

1. i would also like to add information how to use multiple extensions with the same database! (once i figure it out myself)
2. add invitation to develop RDFa and Microdata extensions... conservative folks could also work on RDF/XML 😆 (of course Microdata wouldn't go under RDF)

[elf-pavlik]

looks like i can use multiple extensions like that:

var levelgraph = require('levelgraph');
var lgN3 = require('levelgraph-n3');
var lgJSONLD = require('levelgraph-jsonld');

var db = lgJSONLD(lgN3(levelgraph('test')));

for more than two at the same time it may look weird, i quite like API used in express with .use()

var db = levelgraph('test');
db.use(lgN3());
db.use(lgJSONLD());

[mcollina]

Please, add the various suggestions inside the TODO section :).

As for the extension method, it's based on JS properties and it's the way
of levelup. I'm not really planning to change it, but the express use()
method is a nice API.
Moreover, I'd love to be more levelup-friendly and namespace levelgraph
inside a graph property of levelup (this would allow to have multilevel
support).

[elf-pavlik]

does order of nesting extensions matter in any way?

var db = lgJSONLD(lgN3(lg('test')));
// gives same result as
var db = lgN3(lgJSONLD(lg('test')));

if so i'll just add this additional information to README for now...

[mcollina]

The order matter if an extension depend on another one: lgn3 and lgjsonld
are independent.

The extensions are nested in a prototype chain.

[elf-pavlik]

@mcollina i don't have anything more to add here at this moment!

[mcollina]

Merged, thanks!