README.md: Clarify (lack of) MSRV policy
README.md: Clarify (lack of) MSRV policy

MSRV policy is still being discussed on an ongoing basis in
https://github.com/rust-lang/libs-team/issues/72 . In the interim,
clarify in README that libc doesn't currently have an MSRV policy.

rust-lang/libc,rust-lang/libc,rust-lang/libc