Added preliminary extension API description in README
autoconf now checks for guile, and automake builds with guile support.
updated .gitignore to (hopefully) ignore files generated by the build system.

will probably need to stay on top of this better unless there's another better
way of doing it. As I add features, there may be more build files generated
at build time that I don't know about. TODO: find out if there is any way to
completely remove a file from the git repo's history so it doesn't need to be
downloaded: for example, a really big texture file that is not needed.