[#239] Suggest storing the contract metadata on IPFS

[rinn7e]

Description

Problem: Currently we suggest storing the contract's TZIP-16 metadata on a separate on-chain contract, however it's getting more common to store it on IPFS for many tokens.

Solution: Mention IPFS in the specification and readme.

Related issue(s)

Resolves #239

:white_check_mark: Checklist for your Pull Request

Related changes (conditional)

• Tests

  • [x] If I added new functionality, I added tests covering it.
  • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

Stylistic guide (mandatory)

• [x] My commits comply with the following policy.
• [x] My code complies with the style guide.

[rinn7e]

Took me a while to understand how IPFS work and how it is compared to the traditional way (torrent).

Since it is pretty straightforward to uploaded the file (simply download the client and upload a file), I only mention the IPFS website in the docs instead of writing a full tutorial.