document api

[michielbdejong]

The readme only gives an example of how to use this plugin as an admin, to listen to all ledger messages. Even if this package just implements the standard ILP plugin API, we should still document what the options for constructor arguments are.

[michielbdejong]

Right now, what I'm doing is something like this:

var Plugin = require('ilp-plugin-bells');
var request = require('request');

function getPlugin(host, user, pass) {
  return new Promise(resolve => {
    request({
      method: 'get',
      uri: `https://${host}/ledger`,
      json: true,
    }, (err, sendRes, body) => {
      resolve(new Plugin({
        prefix: body.ilp_prefix,
        account: `https://${host}/ledger/accounts/${user}`,
        password: pass,
      }));
    });
  });
}