Document dependency lifetimes

[meadsteve]

There are broadly three lifetimes:

• Singleton - exists for the life of the container once created.
• Invocation level shared - constructed once during a single function call (wrapped by a lagom decorator).
• Regular - Constructed new every time they are needed.

Special case for a lot of the framework integrations:

• Request level - This is equivalent to the invocation level shared resources above but specifically for the lifetime of a request.