Add a deprecation warning

[rylev]

This crate is no longer being actively maintained as all of the work in supporting COM is going into windows-rs. This should hopefully make that clear.

Closes #206

[MarijnS95]

https://doc.rust-lang.org/cargo/reference/manifest.html#the-badges-section

It seems that the [badge] maintenance = { status = "deprecated" } system is deprecated, could we have a similar badge on top of the readme though?

EDIT: I don't see such a badge being used anywhere, but I wonder if things like cargo-deny source from it?
EDIT2: Or is that nowadays submitted to https://github.com/RustSec/advisory-db?

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Side-note: On the one hand glad to see efforts focused on a single system (windows-rs has full support for COM, no need to have competing implementations), but on the other it's sad to see this go. So long, com-rs!