some config issues on Debian 8

[tbrowder]

Oops, I didn't read the README.md closely enough, sorry, and I'm closing the issue! However, I still think an INSTALL file with the configuration info from the README.md would be useful.

I wasn't informed about the following config issues until running acme-client with real inputs:

1. non-existent directory "/var/empty"

2. the DEFAULT_CA wasn't correct, and I'm still investigating how to change it

I think an INSTALL file with such details would be helpful.

Thanks for an excellent and much needed project!

BTW, libseccomp failed the live tests on my host.

[tbrowder]

I just found that the README.md is NOT in the source directory of the stable version, so I believe my original issue is still valid.