improve phpcr loader doc

[dbu]

had a hard time figuring out that things where not working anymore because the signature of the phpcr loader changed...

is there a reason why this bundle does not allow to configure the phpcr loader service but recommends defining your own service?

see also symfony-cmf/media-bundle#3 - i expect somebody will soon attempt to improve the phpcr loader support.

[lsmith77]

So far we just left the bundle config simpler by not trying to support all possible loaders .. especially i expect a lot of customization of the services. however i am not opposed to making the out of the box experience nicer by supporting some sensible defaults.

[dbu]

if we where going through a the bundle configuration, we would have noticed the config is incomplete. this way, i had to realize why i just get no image... but yes, it would make the config more complicated, but as it will be optional sections i think that would not be too bad.
lets see when/if it happens...