Added note recommending ServiceManager

A number of blog posts have been created benchmarking DI containers over the
past few years. `Zend\Di` always fares poorly -- but we're well aware of that
issue, and the ServiceManager actually exists _because_ we're aware of the
issue.

Additionally, most DI container needs are more general IoC needs, making these
benchmarks largely pointless.

The note added discusses the decisions for using `Zend\Di`, and why you might
want to choose the ServiceManager instead.