Adds documentation for custom binary URLs

[JoshVanL]

What this PR does / why we need it:
Adds some documentation on how to use custom URLs for various tools and binaries in tarmak.

fixes #393

NONE

/assign @simonswine

[jetstack-bot]

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[jetstack-bot]

@JoshVanL: PR needs rebase.

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[JoshVanL]

/close

[jetstack-bot]

@JoshVanL: Closing this PR.

In response to this:

/close

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