Documentation: explain why APCu is not available

[mnapoli]

APCu does not make sense on lambdas because each lambda boots a new CLI process (there is no shared memory like with PHP-FPM), this needs to be explained in the documentation to avoid surprises.

The idea would be to recommend instead to use a shared cache like Memcached or Redis (AWS supports both for example).

[ghost]

Hi, so what about OPcache too? There is no profit of the pre-compilation with Lambda? :-(
Any way to share the shm between instances?

[mnapoli]

@symcaster Opcache is enabled, see #14

[ghost]

@mnapoli Thank, having opcache enabled is not the same that having its cache shared between requests, but Ok, you post #14 answer to that precise point.

[mnapoli]

OK, then just to be clear for everyone reading this discussion: opcache is enabled and the cache is shared between requests for a single lambda instance.

[mnapoli]

We can close this because with 0.3 we will be able to use APCu with PHP-FPM 🎉