Restore links in README.md
Restore links in README.md

These were deleted accidentally in 073b12ff35a8ec6314d655804915f177ce453227.

emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen