Update usage guide in README to use --builtin-classes.

Usage

-----

For the simplest test-run just pass in the src/ dir of Ext JS 4:

For the simplest test-run just use the `--builtin-classes` option to

automatically produce documentation for JavaScript builtin classes

like Array, String and Object:

    $ jsduck ext-4.0.2a/src --output your/docs

    $ jsduck --builtin-classes --output your/docs

You can also use `--verbose` option to see what's actually happening.

To generate docs for Ext JS 4 add path to the corresponding src/ dir:

    $ jsduck --builtin-classes --output your/docs  extjs-4.0.2a/src

Running JSDuck with the current ext-4.0.2a release is expected to

generate a lot of warnings.  Because of the bugs in doc-comments a

global class will also get created.  You can disable this by adding

`--ignore-global` switch.  If you are bothered by the excessive amount

of warnings, use the `--no-warnings` switch.  Now it might look like

nothing is happening... check out the `--verbose` flag.  For more

command line options type `jsduck --help`.

of warnings, use the `--no-warnings` switch.  For full list of command

line options type `jsduck --help=full`.

The latest ext-4.0.6 release will produce only few warnings, so use

that if you can get it.

To generate docs for your own project, simply add as many other input

directories as needed:

    $ jsduck ext-4.0.2a/src project1/js project2/js --output your/docs

    $ jsduck --builtin-classes ext-4.0.2a/src project1/js project2/js --output your/docs

Of course you don't have to include the whole Ext JS into your

documentation, but if your project is built on top of it, it makes