`cortex_m::asm::bootload` doesn't mention vector table relocation

[dimpolo]

So I spend a while figuring out why my interrupts weren't working and it seems like I wasn't the first.

I think cortex_m::asm::bootload should at least mention setting VTOR in its documentation. Maybe it could even implement it directly if a set_vtor paramter is true or something similar

See also https://github.com/rust-embedded/cortex-m-rt/pull/338