README.md: update

This documentation is outdated. We have since moved the code to GitHub
and modernized some of the options for the scripts as well as added new
features. Bring the documentation up to date and make it clearer and a
bit less redundant.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
.gitignore: ignore temp files created by compiling the library

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>