Document build instructions

[NickHu]

Add a README; give a brief overview of the build tools we are using

[jamievicary]

It would be great if someone could do this. I wanted to try to build the new code this afternoon, but couldn't really get started.

[NickHu]

The basic build instructions are as follows (I just tested this on a fresh Windows machine, will write up properly later):

1. Install purescript compiler and spago build tool

• There is a shell.nix provided for Nix
• On Windows, the recommended way of doing this is via npm install -g purescript spago

2. In the relevant directory, e.g. core or webclient, run spago build

The compiled code can be played around with in the repl (spago repl); currently, there isn't much of a user interface.

[jamievicary]

Thanks, it's working.