Suggested build instructions

[stain]

My attempt to address issue #1

See Markdown rendering on https://github.com/stain/sunzip/blob/build-instructions/README.md

Obviously build instructions will vary alot by OS, but I thought an example for Debian/Ubuntu might be helpful, in particular as contrib/blast and contrib/infback9 are not compiled (or even included) in their outdated zlib.

I probably got half the gcc options wrong.. please help! :) Perhaps it should compile libz as a static library to avoid the -rpath trick? Or should one simply compile the contribs against the older system zlib1g-dev (1.2.11 in Ubuntu 18:04, 1.2.8 in Debian 9)?

I also added a Dockerfile to create a Docker image (to test my instructions). The instructions to build that suggest madler/sunzip as Docker image name. You could add an automated build on https://cloud.docker.com/ which would pick up this Dockerfile and re-build on any changes here. To test it today try my test-build stain/sunzip.