document download server's API in installation docs

[jennifer-shehane]

cypress-io/cypress#866 (comment)

These are all accessed on host: https://download.cypress.io

Desktop App Routes

GET /desktop                   ## downloads latest desktop app
GET /desktop?platform=p        ## downloads latest desktop app for specific platform
GET /desktop.json              ## returns desktop manifest.json
GET /desktop/:version          ## downloads desktop app by version
GET /desktop/:version?platform=p    ## downloads desktop app by version for specific os

You don't need to provide platform as Cypress will automatically figure that out. It also supports an arch query param but that's not useful until we build 32 and 64 bit binaries.