Readme doesn't include the `--add-rpath` option

[podiki]

I, at least, did not even know about the --add-rpath, until I searched this repo for something similar. I had looked at the readme but didn't see this feature, but luckily searched. Of course, a simple patchelf --help also shows it often I'm looking at a project's repo to start with.

So, simple suggestion to put this in the readme (and anything else that might not be up to date), as usually the first point of contact for a prospective user.