Add documentation/links to indicator definitions/papers

[MathewBiddle]

It would be great to include a link to the definition of each of the indicators in this package. For example, https://obis.org/indicators/documentation/.

Probably add it to the readme and documentation webpage.

[7yl4r]

Pieter's recommended citations for ES50:

• https://esajournals.onlinelibrary.wiley.com/doi/10.2307/1934145
• https://www.sciencedirect.com/science/article/pii/B978012384719500424X

[7yl4r]

related: #38