Update docs with frame explanation

[vinayak-mehta]

Is your feature request related to a problem? Please describe.

Opening this issue based on the Gitter conversation.

Describe the solution you'd like

I think the docs here could be updated with a better explanation of where frames lie in the Screen/Scene/Effect hierarchy, how they're different from the Frame object, along with an example of how we can use start_frame and stop_frame to time changes inside a Scene.

Describe alternatives you've considered

NA

Additional context

NA

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Thanks again for asciimatics!

[peterbrittain]

Thanks. I'll try to fix that this week.

In the meantime, please feel free to submit your current code for #263 and we can discuss tweaks in that PR.

[vinayak-mehta]

Thanks! I'll submit the current code on #263 today 👍