Document that you should Initialize all objects before entering application

[biniona]

Document that minject is intended to initialize all objects before you start your application.

This is largely due to the fact that the registry must lock while objects are initialized, which can cause slow start-up times and could cause issues if say a health check passes but objects still need to initialize slowly.

[xiyan128]

from slack

We may drop #20 depending on how thread safety ticket goes as well.
@biniona