Adding withdrawals and challenges in plasma cash docs

[nanspro]

• I have read and understood the Plasma Group Contributing Guide and Code of Conduct and am following those guidelines in this pull request.

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Fixes #78

Description

Explaining what invalid withdrawals are and how they're challenged?

[nanspro]

@kfichter this PR doesn't explain the challenges because it has already been explained in #189.
Let me know if i have to add them here also.

[ChrisChinchilla]

@nanspro I made some updates in fork of your fork - nanspro#1

@kfichter What do you think?

[nanspro]

Thanks @ChrisChinchilla !!

[nanspro]

@ChrisChinchilla any update on this, as i said challenges are explained thoroughly in the other PR so let me know if i have to do anything on this one!!

[smartcontracts]

Heyo! We're doing some repo overhauls and our docs are moving around. We're going to close this PR until we can find the right place to put this information.