docs: update cortex-m-rt version requirement

[japaric]

generated crates are also compatible with v0.6.x of cortex-m-rt

[japaric]

r? @rust-embedded/tools (anyone)
cc @rust-embedded/cortex-m

[japaric]

cc @rust-embedded/cortex-m

to elaborate: there's no need to regenerate the device crate; simply update the version requirement in Cargo.toml.

[japaric]

This turned out to be more complicated than I thought it to be.

It is true that device crates are compatible with both v0.5.x and v0.6.x. However, specifying ">=0.5.0, <0.7.0" as the version requirement of a device crate causes Cargo to compile both a v0.5.x version and a v0.6.x version when using that crate with cortex-m-rt v0.5.x. In other words; Cargo won't try to reduce the number of copies of cortex-m-rt in the dependency graph. Having two copies of cortex-m-rt leads to the compiler error we added in rust-embedded/cortex-m-rt#106 so this version requirement actually causes more problems.

[therealprof]

Yeah, tell me about it... I've invested many hours already updating my few crates for compatibility with the latest cortex-m-rt version.

[japaric]

closing in favor of #235