Update README on automatically include new files

[jc4x4]

See discussion in #22.

[jc4x4]

Thanks!

For the regression test use case, I am not sure that this is an appropriate recommendation. It would cause the library to be rebuilt every time any test is modified, which is bad in two ways:

* The lib target is getting rebuilt, not just the test target as would be necessary.

* The unneeded rebuild occurs on every modification, not just on newly added files, so this can severely affect the inner loop of iterating on those files.

For the regression test case, I expect that when a new regression test is added, the next cargo test should exercise it. Without a build script change, the user would have to manually do a clean build, which isn't very intuitive and obvious.

It would certainly be nice if there is a way to only rebuild for new file, not modification. If there are too many rebuilds, one can isolate the regression test cases into a separate crate. That way, the rebuild only affects what's inside the crate.