Merge pull request #32 from zec/using-libclang

Update to USING doc about `libclang` dep

USING.md

@@ -8,7 +8,10 @@ This is what you need to do:
 
 ## Prerequisites
 
-First off, you need to [install Rust], if you haven't done so already.
+First off, you will need the C toolchain and other build tools (_e.g._, CMake)
+you are using to build the rest of your cFS system.
+
+Next, you need to [install Rust], if you haven't done so already.
 Make sure the `cargo` tool is in your `$PATH`.
 
 For the time being, we need the `nightly` release channel
@@ -17,16 +20,27 @@ If you're using `rustup` to manage your Rust installation, this will add
 that channel:
 
 ```sh
-$ rustup toolchain install nightly
+rustup toolchain install nightly
 ```
 
 We also need the Rust standard library's source code
 (for reasons relating to how we handle panics):
 
 ```sh
-$ rustup +nightly component add rust-src
+rustup +nightly component add rust-src
+```
+
+Finally, you will need to install `libclang` and the associated development files,
+as they are required for automated generation of low-level bindings to cFS C APIs.
+For example, on most Debian-like systems, you can run the following (as `root`):
+
+```sh
+apt install libclang-[n]-dev libclang1-[n]
 ```
 
+where `[n]` is some version of `clang` (generally, the newer the better,
+but [version 5 at minimum]).
+
 ## Build system additions
 
 The `cfs-sys` crate,
@@ -185,6 +199,7 @@ You can find a fully worked-out example of a Rust-using cFS application at
 [install rust]: https://www.rust-lang.org/tools/install
 [1]: https://rust-lang.github.io/rustup/concepts/channels.html
 [2]: https://doc.rust-lang.org/book/appendix-07-nightly-rust.html
+[version 5 at minimum]: https://rust-lang.github.io/rust-bindgen/requirements.html
 [a little configuration]: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-reads
 [shared compile caches]: https://doc.rust-lang.org/cargo/guide/build-cache.html#shared-cache
 [sccache]: https://github.com/mozilla/sccache