Add docs about locateResource

[weaverryan]

This was a suggestion from a friend of mine (@tacman) about the missing docs for Kernel::locateResource:

$kernel = $container->getService('kernel');
$path = $kernel->locateResource('@AcmeDemoBundle/path/to/file/Foo.txt');

Additionally, it was interesting that he said he was first looking in the "Filesystem component docs" for something like this, and then later in the "Finder component docs". That's something to think about - those both make logical sense, but obviously aren't the right places to look. It makes me wonder how we can organize things better.