Improve MSRV documentation

[jschwe]

It looks like we haven't documented our minimum supported Rust version in the Readme. We should probably add a table describing what the minimum supported versions are.

• The minimum supported Rust version for compiling the Generator
• The minimum supported Rust versions for compiling user code. This should probably be done as a table, since it may vary by platform.

[ogoffart]

Should we really have a different MSRV for the generator and the user code? One without the other doesn't really make sense.

[jschwe]

I guess it does make the documentation more complex, but fundamentally you can compile the generator on a different machine and ship it together with the corrosion cmake files to the computer actually compiling the project.

For me the main difference between the two is that the MSRV for the generator is pretty well defined, because it either compiles or does not with that version and we can enforce that pretty easily. For user code, our MSRV is more like a guideline or an expectation of what works, and is mostly due to some cargo bugs that eventually got fixed. I would assume that for a no_std application corrosion actually should be able to support pretty old versions of rust.