write docs related to providing debug info for crashes

[radhermit]

Inevitably some third party will crash the underlying rust code in some fashion at which point it would be nice to have documented ways for extracting better debug output for those not familiar with rust and/or C.

For example, setting RUST_BACKTRACE=1 in the env and/or using gdb to run a python script using the python bindings to provide a backtrace (assuming debug symbols aren't stripped from the pkgcraft-c library).