Tigon README.md

Draft of README.md file. This is the file that appears as the "home page" for the GitHub repo.
You need libcurl at runtime. You need the curl/curl.h headers, provided by `libcurl-devel` on RHEL systems and `libcurl-dev` on Debian-based systems. Users who provide curl via compilation (download curl tarball, ./configure && make && make install) will have the headers provided during make install.