Documentation for RLPList

[mkeen]

- What I did

Fixes: #606
Added documentation for RLPList.

- How I did it

Through research on another issue I learned a little bit about how to use RLPList, and noticed this issue, so I documented what I learned.

- How to verify it

Run make docs and open ./docs/_build/html/built-in-functions.html

- Description for the changelog

Added documentation for RLPList

- Cute Animal Picture

[jacqueswww]

@mkeen could you add notes in the documentation that RLP decoder needs to be deployed for this.
We will eventually create a fixed address for the decoder on mainnet, however the decoder in the code currently used only for the test suite (see https://github.com/ethereum/vyper/blob/master/vyper/utils.py#L110), but it's important in the docs to note that we require it ;)

If one tests it outside the testsuite the transactions will fail.

[mkeen]

@jacqueswww Added some notes. Let me know if you had something else in mind. Happy to make any changes necessary :)