code: public Document append(final String key, final Object value) {
          documentAsMap.put(key, value);
          return this;
      }
docs: Put the given key/value pair into this Document and return this.  Useful for chaining puts in a single expression, e.g.
  <pre>
  doc.append("a", 1).append("b", 2)}
  </pre>
  @param key   key
  @param value value
  @return this