Describe the interface of the Overview tab in the README.

[luisdev]

The README.md file should describe the various graphs on the Overview tab and what they mean.

For example, what does it mean when the memory graph shows a high value, like this:

And what does it mean when it shows a low value, like this:

[amnuts]

OK, I'll consider this. I always just thought it was fairly obvious; you have a graph called "memory" and either that memory is 97% full or 7% full (or 3% empty and 93% empty, depending on your frame of reference).

This also ties in with the settings, opcache.memory-consumption and the amount of memory reported as used (shown under the "interned strings usage" header).