Add uefi/README.md (#577)

This is mostly copied from the top-level readme, with some edits to
focus more on the `uefi` package rather than the whole project.

Author: nicholasbishop

uefi/README.md

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+# uefi-rs
+
+[![Crates.io](https://img.shields.io/crates/v/uefi)](https://crates.io/crates/uefi)
+[![Docs.rs](https://docs.rs/uefi/badge.svg)](https://docs.rs/uefi)
+![Stars](https://img.shields.io/github/stars/rust-osdev/uefi-rs)
+![License](https://img.shields.io/github/license/rust-osdev/uefi-rs)
+![Build status](https://github.com/rust-osdev/uefi-rs/workflows/Rust/badge.svg)
+
+[UEFI] is the successor to the BIOS. It provides an early boot environment for
+OS loaders, hypervisors and other low-level applications.
+
+The `uefi` crate makes it easy to:
+- Write UEFI applications in Rust (for `i686`, `x86_64`, or `aarch64`)
+- Call UEFI functions from an OS (usually built with a [custom target][rustc-custom])
+
+The objective is to provide **safe** and **performant** wrappers for UEFI interfaces,
+and allow developers to write idiomatic Rust code.
+
+Check out the [UEFI application template] for a quick start.
+
+[UEFI]: https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface
+[rustc-custom]: https://doc.rust-lang.org/rustc/targets/custom.html
+[UEFI application template]: https://github.com/rust-osdev/uefi-rs/tree/HEAD/template
+
+## Optional features
+
+- `alloc`: Enables functionality requiring the `alloc` crate from the Rust standard library.
+  - For example, this allows many convenient `uefi-rs` functions to operate on heap data (`Box`).
+  - It is up to the user to provide a `#[global_allocator]`.
+- `global_allocator`: implements a `#[global_allocator]` using UEFI functions.
+  - This allows you to use all abstractions from the `alloc` crate from the Rust standard library
+    during runtime. Hence, `Vec`, `Box`, etc. will be able to allocate memory.
+    **This is optional**, so you can provide a custom `#[global_allocator]` as well.
+  - There's no guarantee of the efficiency of UEFI's allocator.
+- `logger`: logging implementation for the standard [`log`] crate.
+  - Prints output to UEFI console.
+  - No buffering is done: this is not a high-performance logger.
+  
+See also the [`uefi-services`] crate, which provides a panic handler and
+initializes the `global_allocator` and `logger` features.
+
+[`log`]: https://github.com/rust-lang-nursery/log
+[`uefi-services`]: https://crates.io/crates/uefi-services
+
+## Documentation
+
+The [uefi-rs book] contains a tutorial, how-tos, and overviews of some
+important UEFI concepts.
+
+Reference documentation can be found on docs.rs:
+- [docs.rs/uefi](https://docs.rs/uefi)
+- [docs.rs/uefi-macros](https://docs.rs/uefi-macros)
+- [docs.rs/uefi-services](https://docs.rs/uefi-services)
+
+For additional information, refer to the [UEFI specification][spec].
+
+[spec]: http://www.uefi.org/specifications
+[uefi-rs book]: https://rust-osdev.github.io/uefi-rs/HEAD
+
+## Building UEFI programs
+
+For instructions on how to create your own UEFI apps, see [BUILDING.md].
+
+The uefi-rs crates currently require some [unstable features].
+The nightly MSRV is currently 2022-08-08.
+
+[unstable features]: https://github.com/rust-osdev/uefi-rs/issues/452
+[BUILDING.md]: https://github.com/rust-osdev/uefi-rs/blob/HEAD/BUILDING.md
+
+## License
+
+The code in this repository is licensed under the Mozilla Public License 2.
+This license allows you to use the crate in proprietary programs, but any modifications to the files must be open-sourced.
+
+The full text of the license is available in the [license file](LICENSE).