update README for crates.io

Author: steveklabnik

README.md

@@ -4,3 +4,19 @@ rlibc
 A bare minimum "libc" for Rust crates that do not want to rely on libc itself.
 This crate provides functions which LLVM often lowers intrinsic calls to and
 will be required to link correctly.
+
+## Usage
+
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+
+rlibc = "*"
+```
+
+And add this to your crate root:
+
+```rust
+extern crate rlibc;
+```