Add installation instructions

[tennox]

You could add some instructions on how to build/install this to the readme.

[w-p]

make
mv ./su-exec /usr/local/bin

[grosser]

one-liner I use for docker:

curl -L https://github.com/ncopa/su-exec/archive/dddd1567b7c76365e1e0aac561287975020a8fad.tar.gz | tar xvz && \
cd su-exec-* && make && mv su-exec /usr/local/bin && cd .. && rm -rf su-exec-*