Document `uparse`

[briochemc]

Currently, searching for uparse in the docs (dev) gives one unrelated result:

Maybe an example of using uparse could be added to the docs?

Also related to the question I just asked in #214, maybe add a function that does the reverse of uparse, i.e., that takes a Quantity and returns a valid (uparse-able) String?

[michikawa07]

I have created a package that looks like this
https://github.com/michikawa07/UnitfulParsableString.jl

I also have a pull request for the contents of this package.