filesystem example (#2236)

docs/examples-rust-wasi-fs.md

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+# WASI
+
+You can also [browse this source code online][code] and clone the wasmtime
+repository to run the example locally.
+
+[code]: https://github.com/bytecodealliance/wasmtime/blob/main/examples/wasi/main.rs
+
+This example shows off how to run a wasi binary with a memory filesystem.
+
+## Wasm Source code
+
+```rust,ignore
+{{#include ../examples/wasi-fs/wasm/wasi-fs.rs}}
+```
+
+
+## `wasi-fs.rs`
+
+```rust,ignore
+{{#include ../examples/wasi-fs/main.rs}}
+```