code: public RestNode addCustomProperty( String name,
                                         String value ) {
          customProperties.put(name, value);
          return this;
      }
docs: Adds a custom property to this node, meaning a property which is not among the standard JCR properties
  
  @param name a {@code non-null} String, representing the name of the custom property
  @param value a {@code non-null} String, representing the value of the custom property
  @return this instance, with the custom property added