Prepare docs for libc v0.3

Author: JohnTitor

CONTRIBUTING.md

@@ -3,6 +3,13 @@
 Welcome! If you are reading this document, it means you are interested in contributing
 to the `libc` crate.
 
+## v0.2 changes
+
+If you want to add your changes to v0.2, please submit them to the `libc-0.2` branch.
+If you want to add any breaking changes, it should be submitted to the main branch,
+which has changes for v0.3.
+We will support and make a new release for v0.2 until we make the first release of v0.3.
+
 ## Adding an API
 
 Want to use an API which currently isn't bound in `libc`? It's quite easy to add

README.md

@@ -16,6 +16,15 @@ More detailed information about the design of this library can be found in its
 
 [rfc]: https://github.com/rust-lang/rfcs/blob/HEAD/text/1291-promote-libc.md
 
+## v0.3 Roadmap
+
+The main branch is now for v0.3 which has some breaking changes.
+
+For v0.2, please submit PRs to the `libc-0.2` branch instead.
+We will stop making new v0.2 releases once we release v0.3 on crates.io.
+
+See the [tracking issue](https://github.com/rust-lang/libc/issues/3248) for details.
+
 ## Usage
 
 Add the following to your `Cargo.toml`: