Include installation instructions in the README

[ELLIOTTCABLE]

This isn't an issue I've ever had with an open-source project before, heh, but …

how do I install nsjail?

In particular, it'd be great if there were the usual install-instructions-per-distro in the README — something like apt/snap/donwload tarball/build from source.

(Related: #137!)

[goldstar611]

The build instructions are quite short and are noted in the Dockerfile at https://github.com/google/nsjail/blob/3932b67781d3e15bfefc0cccfda6a6884a2677cc/Dockerfile

Basically, in an Ubuntu virtual machine, run

sudo apt update
sudo apt install autoconf bison flex gcc g++ git libprotobuf-dev libnl-route-3-dev libtool make pkg-config protobuf-compiler

git clone https://github.com/google/nsjail.git

cd nsjail

make

sudo mv /nsjail/nsjail /usr/local/bin/

That last step makes nsjail available in your path so any app or command prompt will be able to find it.