document the minimum supported Rust version (policy)

[tanriol]

I tried to find information on the minimum Rust version bindgen supports (MSRV) and failed to find this information or any policy regarding this, including the readme, the book's Requirements and FAQ sections. Could you please document it somewhere?

[emilio]

That's fair. We should:

• Figure out what the current MSRV is.
• Document it.
• Test it so that we don't accidentally regress it.

I'm still a bit on the fence on whether bumping MSRV should be a breaking change in the library or not, though the above steps should be uncontroversial.

[highfive]

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