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.
Merge branch '#1336-note-about-zend-servicemanager-over-zend-di'

Close #1336