Add documentation for imports and input filters

[nickrsan]

Original report by Nicholas Santos (Bitbucket: nickrsan, GitHub: nickrsan).

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Importing and Input filters don't have decent documentation. Make that. Need documentation on adding a new input filter

[nickrsan]

Original comment by Nicholas Santos (Bitbucket: nickrsan, GitHub: nickrsan).

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Done as well as it's going to be for 2.0 Could use more work in the future, but all of the code can.