add shell.nix
nix-shell provides a way to have isolated development environments.
Zoekt's development requirements are pretty vanilla: go and git.
However, I sometimes have issues with getting universal-ctags installed
across different environments I run.
I've seen some other go projects adopt the use of nix-shell, in
particular a few of the hashicorp projects. I've ended up using this for
a while now whenever developing in zoekt and want to share for others.
Change-Id: Icfcc3c7461565fed19aea607b04e03e67b42ce05