USING: added libclang prerequisite (BlackCAT-CubeSat#31)

Someone recently ran into this issue when getting started.
Hopefully this change will keep others from running into the same problem.

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
@@ -27,6 +30,16 @@ We also need the Rust standard library's source code
 $ 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 a sufficiently recent version of `clang` (at time of writing, it must be at least `5`).
+
 ## Build system additions
 
 The `cfs-sys` crate,