Update docs for contributors

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,11 +1,14 @@
 Thanks for considering submitting a PR!
 
+We have the [contribution guide](https://github.com/rust-lang/libc/blob/main/CONTRIBUTING.md). Please read it if you're new here!
+
 Here's a checklist for things that will be checked during review or continuous integration.
 
-- \[ ] Edit corresponding file(s) under `libc-test/semver` when you add/remove item(s)
-- \[ ] Your PR doesn't contain any *unstable* values like `*LAST` or `*MAX` (see [#3131](https://github.com/rust-lang/libc/issues/3131))
-- \[ ] If your PR increments version number, it must not contain any other changes
-- \[ ] `rustc ci/style.rs && ./style src`
+- \[ ] Edit corresponding file(s) under `libc-test/semver` when you add/remove item(s), e.g. edit `linux.txt` if you add an item to `src/unix/linux_like/linux/mod.rs`
+- \[ ] Your PR doesn't contain any private or *unstable* values like `*LAST` or `*MAX` (see [#3131](https://github.com/rust-lang/libc/issues/3131))
+- \[ ] If your PR has a breaking change, please clarify it
+- \[ ] If your PR increments version number, it must NOT contain any other changes (otherwise a release could be delayed)
+- \[ ] Make sure `ci/style.sh` passes
 - \[ ] `cd libc-test && cargo test`
   - (this might fail on your env due to environment difference between your env and CI. Ignore failures if you are not sure)
 

CONTRIBUTING.md

@@ -54,7 +54,7 @@ We have two automated tests running on [GitHub Actions](https://github.com/rust-
   - `cd libc-test && cargo test`
   - Use the `skip_*()` functions in `build.rs` if you really need a workaround.
 2. Style checker
-  - `rustc ci/style.rs && ./style src`
+  - [`sh ci/style.sh`](https://github.com/rust-lang/libc/blob/main/ci/style.sh)
 
 ## Breaking change policy