Reworks ffi enablement documentation & adds context to exception

[nfreader]

Hello!

I ran into an issue where I assumed ffi was properly enabled; it was not and I spent many hours trying to debug the problem. This PR reworks the section of the readme that talks about enabling FFI. I've also added an ini_get check in the FFI class that handles loading libraries. Right now it just appends to the message thrown by the exception, but it might work better as its own exception.

[jcupitt]

How about moving this up a bit? On line 284 we have:

        if (!ini_get('ffi.enable')) {
            throw new Exception("ffi.enable not set to 'true'");
        }

We could change that to:

        if (!ini_get('ffi.enable') ||
            ini_get('ffi.enable') === 'preload') {
            throw new Exception("ffi.enable not set to 'true'");
        }

I think (I think??) we need both tests.

[nfreader]

That’s probably a better place for it, I just worry about unintended consequences. I don’t know enough about FFI to fully grasp what this might mean later on down the line.

[jcupitt]

Ahh how annoying, I'm sorry that bit you. Yes, happy to accept this PR.