Added filesystem access example (#55)

Adds a filesystem access example to `README.md`. 

I'm a little unsure what `__wasm_call_ctors()` and `_initialize()` do
and may need a better worded explanation as part of the example. My
assumption is that it initializes some needed runtime APIs needed for
filesystem access.

Let me know of any adjustments needed.

README.md

@@ -265,6 +265,57 @@ config := PluginConfig{
 _, err := NewPlugin(ctx, manifest, config, []HostFunction{})
 ```
 
+### Enable filesystem access
+
+WASM plugins can read/write files outside the runtime. To do this we add `AllowedPaths` mapping of "HOST:PLUGIN" to the `extism.Manifest` of our plugin.
+
+```go
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	extism "github.com/extism/go-sdk"
+)
+
+func main() {
+	manifest := extism.Manifest{
+		AllowedPaths: map[string]string{
+			// Here we specifify a host directory data to be linked
+			// to the /mnt directory inside the wasm runtime
+			"data": "/mnt",
+		},
+		Wasm: []extism.Wasm{
+			extism.WasmFile{
+				Path: "fs_plugin.wasm",
+			},
+		},
+	}
+
+	ctx := context.Background()
+	config := extism.PluginConfig{
+		EnableWasi: true,
+	}
+	plugin, err := extism.NewPlugin(ctx, manifest, config, []extism.HostFunction{})
+
+	if err != nil {
+		fmt.Printf("Failed to initialize plugin: %v\n", err)
+		os.Exit(1)
+	}
+
+	data := []byte("Hello world, this is written from within our wasm plugin.")
+	exit, _, err := plugin.Call("write_file", data)
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(int(exit))
+	}
+}
+```
+
+> *Note*: In order for filesystem APIs to work the plugin needs to be compiled with WASI target. Source code for the plugin can be found [here](https://github.com/extism/go-pdk/blob/main/example/fs/main.go) and is written in Go, but it could be written in any of our PDK languages.
+
 ## Build example plugins
 
 Since our [example plugins](./plugins/) are also written in Go, for compiling them we use [TinyGo](https://tinygo.org/):