Add docs for Distributions.jl

[slwu89]

To address #615, update guides/add_multimensional_noise.md. I felt that the subject was essentially the same as this existing help file, and would rather add to it than create another one with a similar name. I listed two methods, sampling N points from the multidimensional space, and exhaustive enumeration. I used IterTools.product to generate the product space, I think this should be fairly minimal as a dependency as it has no dependencies of its own, and AFAIK is close to being a "base package" for Julia.

Closes #615

[odow]

I used IterTools.product

Base.product is even simpler :smile:

[odow]

I made a few simple changes. Mainly just formatting. I think this should be pretty helpful. Now that I think about it, I've answered a similar question too many times. I always assumed that it was pretty obvious that you could use distributions and sample. But this makes things a lot clearer.

Any suggestions for other changes?

[slwu89]

Did not know about Base.product, that's fantastic. No other suggestions, thanks!