add example to README

Author: soypat

Diff for: README.md

@@ -15,3 +15,55 @@ How to install package with newer versions of Go (+1.16):
 ```sh
 go mod download github.com/soypat/fat@latest
 ```
+
+### Basic usage example
+
+The following example does the following:
+1. Mounts a FAT filesystem to the `fat.FS` type.
+2. Creates a new empty file called `newfile.txt`, replacing any existing file.
+3. Writes `Hello, World!` to that file.
+4. Closes the file to synchronize pending changes to the FAT filesystem.
+5. Opens the file in read mode and reads all of it's contents and prints them to standard output.
+
+```go
+package main
+
+import "github.com/soypat/fat"
+
+func main() {
+	// device could be an SD card, RAM, or anything that implements the BlockDevice interface.
+	device := NewFATDevice()
+	var fs fat.FS
+	err := fs.Mount(device, device.BlockSize(), fat.ModeRW)
+	if err != nil {
+		panic(err)
+	}
+	var file fat.File
+	err = fs.OpenFile(&file, "newfile.txt", fat.ModeCreateAlways|fat.ModeWrite)
+	if err != nil {
+		panic(err)
+	}
+
+	_, err = file.Write([]byte("Hello, World!"))
+	if err != nil {
+		panic(err)
+	}
+	err = file.Close()
+	if err != nil {
+		panic(err)
+	}
+
+	// Read back the file:
+	err = fs.OpenFile(&file, "newfile.txt", fat.ModeRead)
+	if err != nil {
+		panic(err)
+	}
+	data, err := io.ReadAll(&file)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(string(data))
+	file.Close()
+    // Output: Hello, World!
+}
+```