Improve README and library documentation

[atheriel]

This is a work in progress. Do not merge.

This PR improves the documentation of the nbt crate and adds a much more detailed README.md file for the project.

It also contains small changes to improve the ergonomics of working with Blob objects, so that creating/modifying NBT data has a clearer API. These changes are detailed in the new documentation and examples for that struct.