Improve docoumentation

[nirs]

The current readme usage example is outdated and in complete.

• [ ] Why this library is useful
• [ ] How to open an image
• [ ] How to get image info (like qemu-img info)
• [ ] How to convert an image using the convert package (like qemu-img convert)
• [ ] How to convert an image and stream the data using io.Copy
• [ ] Link to relevant examples in cmd/go-qcow2reader-example
• [ ] Benchmarks comparing to qemu-img convert
• [ ] Link to projects using the library