Add an upgrade guide for v2

[grossbart]

I started an upgrade guide for the v1 -> v2 transition, I hope it helps. Depending on how many questions are asked, this guide might evolve.

I hoped to do this earlier, but time is sparse at the moment, sigh …

[dakom]

Until it is merged - I guess it makes sense to add suggestions/questions here?

One that just bit me - seems there is no longer O.getOrElseL(), rather O.getOrElse() is always lazy?

[grossbart]

I'll add a note to the document, the discussion happened here: #823 (comment)

tl;dr having a single API is preferrable, and always lazy is more general.

[giogonzo]

I think this (tsc hanging) will be no longer the case with fp-ts >=2 ? I remember someone mentioning it in multiple occasions, but never verified myself. cc @gcanti

[leemhenson]

If all the versions you have installed at >= 2 then I think the issue will no longer occur.

[grossbart]

I think you're right, this was one of the motivations behind the removal of classes.

[dakom]

Suggest mentioning (as discussed in #903) that pipe() is helpful, even with only one function, for the following reasons:

1. type inference
2. allows for easier migration since args don't need to be flipped around