README explain cargo run permission for gdb #113

[hazelutf8]

Update README.md to explain how to give cargo run permission to launch gdb.
As discussed in discord and #113, on first-run the gdb debugger doesn't give cargo run permission to run gdb automatically and load the FW to the HW.

This PR updates the README.md to show a command which can give this permission, and limits the permission addition to the scope of the stm32f1xx-hal repository in use.

Because of the different conventions between Linux and Windows, the two commands were unable to be distilled into the same line of text.

It is unknown if only the Linux example is wanted, and the windows version of the command should be removed?
It seems the README.md already makes this assumption elsewhere?

Suggestions are welcome.

[therealprof]

Thanks. It's a clear improvement and we can always tweak it some more later.

[TheZoq2]

It is unknown if only the Linux example is wanted, and the windows version of the command should be removed?

We definitvely should have windows instructions as well if possible. Forutnately, I have been able to avoid using windows for years :P, unfortunately that means that I can't really write instructions for it