Platform support section in Readme out of date

[atouchet]

See ci/build.sh for the platforms on which libc is guaranteed to build for each Rust toolchain. The test-matrix at [Travis-CI], [Appveyor], and Cirrus-CI show the platforms in which libc tests are run.

The Travis and AppVeyor links were removed in #1376.

[atouchet]

Perhaps related but Cargo.toml also still lists:

[badges] travis-ci = { repository = "rust-lang/libc" } appveyor = { repository = "rust-lang/libc", project_name = "rust-lang-libs/libc" }

Should those be removed as well?

[gnzlbg]

Should those be removed as well?

Yes, we should add a badge for azure-pipelines.

[atouchet]

The Readme was fixed in 07d1f7a but Cargo.toml still needs to be updated.

[atouchet]

This was finished in #1687.