chore: First pass at an OmniBOR package README (#141)

This commit adds an initial README for the `omnibor` package, which
encompasses both the OmniBOR library and binary crates.

Signed-off-by: Andrew Lilley Brinker <[email protected]>

omnibor/README.md

@@ -1,4 +1,45 @@
 
-# `omnibor` crate
+# `omnibor` Rust package
 
-TODO(abrinker): Write this.
+This crate implements the [OmniBOR] specification for software identity
+and fine-grained dependency tracking. This means it is intended to provide
+three things:
+
+- __Artifact Identifiers__: independently-reproducible identifiers for
+  software artifacts.
+- __Artifact Input Manifests__: manifests which record all inputs used to
+  produce a software artifact.
+- __Artifact Dependency Graphs__: graphs which represent all known
+  dependencies, at the file level, for constructing a software artifact.
+
+> [!IMPORTANT]
+> The OmniBOR spec, and this Rust package, are still a work-in-progress.
+
+This package defines two crates:
+
+- __Library__: The `omnibor` library, suitable for integrating OmniBOR into
+  your own Rust projects.
+- __Binary__: The `omnibor` CLI, which provides convenient mechanisms for
+  producing and operating with OmniBOR identifiers and manifests.
+
+## Using the Library
+
+Run the following to add the library to your own crate.
+
+```sh
+$ cargo add omnibor
+```
+
+## Using the Binary
+
+Run the following:
+
+```sh
+$ cargo install --path omnibor --features="build-binary"`
+```
+
+## License
+
+All of the OmniBOR Rust implementation is Apache-2.0 licensed.
+
+[OmniBOR]: https://omnibor.io