Add Upgrade guide from 1.0.0 to 2.0.0

[deiga]

Upgrading is quite difficult as the API has changed a fair bit.
Would it be possible to add an upgrade guide?

[evancz]

What are you running into? What sort of things do you think it'd be best to document?

[deiga]

@evancz The changed type signature for all requests was the biggest hurdle I encountered when I tried to do the upgrade.
As well as small things like parameters changing in the custom requests.

I didn't put much effort into it yet, but these felt so big that I just couldn't handle the upgrade right now :P

[bryce13950]

A quick guide on how the types have changed would be great. A simple document that shows 1.0.0 implementation, and then the same implementation in 2.0.0 would probably suffice.

[christelnagels]

My friend @Hermanverschooten made a post on elm-discourse describing how he upgraded my code to 2.0,

[michaelglass]

We just made a shim between 1.0 and 2.0 to help people upgrade from 1.0 to 2.0: https://github.com/NoRedInk/http-upgrade-shim

It provides an, albeit awkward, step-by-step approach to upgrading elm/http.