Added docs for save()/restore()

[diegoperini]

I hope my additions reflect the suggested use cases for save and restore operations.

[diegoperini]

npm install && npm run make failed on my computer, thus Travis is angry with me I guess.

[abitrolly]

@diegoperini did you run them from the docs/? What error have you got?

[abitrolly]

This docker command shall not fail, because it doesn't not depend on the state of local packages.

docker run -v "$(pwd)":"/app" -w "/app/docs" -it --rm node bash -c "npm install && npm run make"

[gajus]

Can this also be used to speed up parser initialization?, e.g.

import nearley from 'nearley';
import grammar from './grammar';
import type {
  Ast,
} from './types';

const rules = nearley.Grammar.fromCompiled(grammar);
const parser = new nearley.Parser(rules);
const state = parser.save();

export const parse = (query: string): Ast => {
  parser.restore(state);

  const results = parser.feed(query).results;

  if (results.length === 0) {
    throw new Error('Found no parsings.');
  }

  if (results.length > 1) {
    throw new Error('Ambiguous results.');
  }

  return results[0];
};