Update README section "Debugging of RustyHermit apps"

[Harry-R]

The README section "Debugging of RustyHermit apps" is outdated and does not work anymore, according to @mkroening .

[mkroening]

I'm happy to report that I was wrong and everything should be working splendidly just like the readme says.

I was thinking about hermit-os/hermit-rs#147 (comment), but my gdbstub rewrite solved that issue for uhyve.

Just remember to use rust-gdb (installed automatically with rust) instead of gdb for a prettier debugging experience.

[mkroening]

Eh, maybe I should update the readme section anyhow. But it should be functional in the current state.

[mkroening]

Ah, to use the GDB support rewrite, install the current master branch with

cargo +nightly install --git https://github.com/hermitcore/uhyve.git --locked uhyve