Need to add documentation for multiple nonprofit fundraisers.

[Bubinga]

When linking to multiple fundraisers, the user needs to use 'special-fundraiser' as the ':nonprofit-identifier' when making a request. For example:

curl "https://partners.every.org/v0.2/nonprofit/special-fundraiser/fundraiser/{fundraiser-name-here}/raised?apiKey=myPublicApiKey"

The same applies when trying to use the Donate Button, they must use this for the link:

Donate