document why default values are always strings

[willkg]

There are a couple of architecture choices that we've made that should get documented so it's clear why it is the way it is.

One that comes up in conversations with people is that default values are always strings that get passed to the parser rather than parsed values. For example 'true' vs. True. I think it's a worthwhile thing to explain in the docs. One nice thing that might come of this is that someone comes up with a better way to meet my requirements.

This issue covers writing that up.

[pjz]

I think https://github.com/willkg/everett/blob/master/everett/manager.py#L874 is enough to let this be closed.

[willkg]

That's buried in the ConfigManager API docs and this issue covers writing something that's easier for everyone to bump into who needs to.