docs: Add instructions for updating wasmtime

Signed-off-by: Ryan Northey <ryan@synca.io>

Author: phlax

bazel/cargo/wasmtime/README.md

@@ -0,0 +1,36 @@
+
+# Updating `wasmtime`
+
+Projects depending upon `proxy-wasm-cpp-host` and `wasmtime` should
+ensure that the `wasmtime` dependency here is updated whenever updating
+`proxy-wasm-cpp-host`.
+
+## Install `rust`
+
+Ensure you have the latest version of rust.
+
+```console
+$ curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+$ rustup update
+```
+
+## Install `cargo-raze`
+
+```console
+$ cargo install cargo-raze --version 0.14.1.
+```
+
+## Generate lockfile
+
+```console
+$ cd bazel/cargo/wasmtime
+$ cargo raze --generate-lockfile
+$ cd ../..
+```
+
+## Commit changes
+
+```console
+$ git add bazel/cargo/wasmtime
+$ git commit ...
+```