Document Bulk Operations in README

Following up from the discussion in https://github.com/share/sharedb/issues/422#issuecomment-755458497 , it would be great to document the bulk operation features of ShareDB in the README.

Here's a start:

const doc1 = connection.get('foo', 'd1');
const doc2 = connection.get('foo', 'd2');

connection.startBulk();
// Issue all the fetches for this bulk request. Could use `async.each` for many docs in an array.
doc1.fetch(() => {});
doc2.fetch(() => {});
// Send all the queued-up doc fetches to the server in a single `bf` request.
// Callbacks for each of the `fetch` calls will be called when the data is received by the client
connection.endBulk();

That sends the fetch to the backend in a single bf (bulk fetch) request, and sharedb-mongo at least will handle fetching the batch of docs with one find() command.

Start with the repository README and the discussion linked from issue #422, then review the provided bulk-operation example and the descriptions of the bf request and callbacks. Done means the README explains how to use connection.startBulk() and connection.endBulk() for multiple fetches, including the relevant behavior of sharedb-mongo.