Update README regarding new warnings.

These can be added using more command line options as explained in the

[Advanced Usage][adv] section of wiki.

Running JSDuck against older Ext JS than 4.0.7 is expected to generate

a lot of warnings.  Similarly your own .js files will probably

generate warnings too.  Sorry for that, JSDuck just wants to be

helpful.  If you are overwhelmed by the warnings, you can disable them

using `--no-warnings` switch.  Another thing that often happens is

that JSDuck is unable to determine into which class a member belongs

and will place all such items into a global class - you can disable

this using the `--ignore-global` switch.  For full list of command

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

Running latest JSDuck is expected to generate lots of warnings.

That's because some warning types were added after Ext JS 4.0.7

release.  Sorry for that, JSDuck just wants to be helpful.  If you are

overwhelmed by the warnings, you can disable them selectively using

something like `--warnings=-link_ambiguous,-no_doc` or you could

disable them all by `--warnings=-all`.

Another thing that often happens is that JSDuck is unable to determine

into which class a member belongs and will place all such items into a

global class - you can disable this using the `--ignore-global`

switch.  For full list of all command line options type

`jsduck --help=full`.

[adv]: https://github.com/senchalabs/jsduck/wiki/Advanced-Usage