Add MIT license

[febo]

This PR adds the MIT license file to the repository. The crate is already licensed under MIT as specified on the Cargo.toml file:

[package]
name = "podded"
version = "0.5.1"
description = "Zero-copy types for constraint environments"
authors = ["Metaplex Developers <dev@metaplex.com>"]
repository = "https://github.com/metaplex-foundation/podded"
license = "MIT"
edition = "2021"
readme = "README.md"

...

And it can be seen on crates.io: