docs: Add initial how-to-build documentation

docs/contributing-building.md

@@ -1,3 +1,61 @@
 # Building
 
-... more coming soon
+This section describes everything required to build and run Wasmtime.
+
+## Prerequisites
+
+Before we can actually build Wasmtime, we'll need to make sure these things are
+installed first.
+
+### The Rust Toolchain
+
+[Install the Rust toolchain here.](https://www.rust-lang.org/tools/install) This
+includes `rustup`, `cargo`, `rustc`, etc...
+
+### `libclang` (optional)
+
+The `wasmtime-fuzzing` crate transitively depends on `bindgen`, which requires
+that your system has a `libclang` installed. Therefore, if you want to hack on
+Wasmtime's fuzzing infrastructure, you'll need `libclang`. [Details on how to
+get `libclang` and make it available for `bindgen` are
+here.](https://rust-lang.github.io/rust-bindgen/requirements.html#clang)
+
+## Building the `wasmtime` CLI
+
+To make an unoptimized, debug build of the `wasmtime` CLI tool, go to the root
+of the repository and run this command:
+
+```shell
+cargo build
+```
+
+The built executable will be located at `target/debug/wasmtime`.
+
+To make an optimized build, run this command in the root of the repository:
+
+```shell
+cargo build --release
+```
+
+The built executable will be located at `target/release/wasmtime`.
+
+You can also build and run a local `wasmtime` CLI by replacing `cargo build`
+with `cargo run`.
+
+## Building Other Wasmtime Crates
+
+You can build any of the Wasmtime crates by appending `-p wasmtime-whatever` to
+the `cargo build` invocation. For example, to build the `wasmtime-jit` crate,
+execute this command:
+
+```shell
+cargo build -p wasmtime-jit
+```
+
+Alternatively, you can `cd` into the crate's directory, and run `cargo build`
+there, without needing to supply the `-p` flag:
+
+```shell
+cd crates/jit/
+cargo build
+```