Add installation instructions to the docs

[Daniel Rodríguez Rivero]

📖 Documentation

I was very surprised to see that there are no installation instructions in the "real" documentation of the project. It may sound silly, but there are many ways to install packages nowadays, so this information is still relevant. For example, I could think about this all possible installation methods:

• npm i fp-ts
• npm i @fp-ts/Option @fp-ts/core
  Etc.
  I suggest to add them quite early in the main page: https://gcanti.github.io/fp-ts/

[Akshit Batra]

@danielo515, Agreed. Seems like a great idea to add the installation instruction.
I feel there is also no page like getting started, which I've seen in a lot of libraries. There it generally consists of installation instructions, pre-requisites and other details.