Add direnv file
Background: I just discovered this awesome utility direnv which lets you simply run some commands when you enter a directory. What makes this so awesome is that it also supports Nix and can load the shell.nix automagically.
With this change, anyone with direnv installed can once run direnv allow . and it will:
- Load the shell.nix (provided nix is installed) which automatically fetches all dependencies
- Run
make env, except without installing anything since you don't want to install redox every time you enter the directory.