Build in a nutshell

Build in a nutshell (for dummies like me).

```
$ ./ci/install.sh
$ source emsdk/emsdk_env.sh
### Alternatively retrieve git submodules (i.e. when cloning argon2-browser repo use flag --recursive) ###
$ git clone https://github.com/P-H-C/phc-winner-argon2 argon2
$ ./build.sh
```

:)