Update index.md to Symfony 3

[mariogalan]

Update index.md to reflect Symfony 3 lock file route

[aschempp]

For Symfony 3.3 this could be updated to %kernel.project_dir%

[gilles-g]

Hi,

'%kernel.cache_dir%' can be an option too

[aivus]

Related to #77

Probably we should consider to use kernel.cache_dir instead of hacks with kernel.root_dir
Also, we need to adjust symfony/recipes-contrib#155 to use kernel.cache_dir

[aivus]

About kernel.cache_dir.
This parameter has environment-dependent value.
That means for dev env it will be /var/cache/dev and for prod - /var/cache/prod.
And lock/unlock will affect only specific env.

So, I'm not sure that it should work in this way.