Readme note that the Go language is required for installation

It's not obvious to new users (i.e. me) that the Go language is required for installation. With MacPorts, I had to install it first before it would work, otherwise I would get this when trying to `make install`:

```
go fmt
make: go: No such file or directory
make: *** [direnv] Error 1
```

The note is also useful because I happen to know the Go language but I know many developers who have never heard of it and so wouldn't have a clue what the problem is.