docs: add README

Author: kxxt

README.md

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+# filterable-enum
+
+Filterable wrapper for discriminated unions(DU) in rust.
+
+## What is this?
+
+In some cases, you may want to filter a DU by a bitmask to check if it matches a subset of all variants.
+This crate provides a derive macro to generate a filterable wrapper for a DU.
+
+Ideally, I think this whole crate should be unnecessary. Rust already stores a tag for representing the variant of a DU.
+However, Rust does not expose this tag to the user, so I cannot really use that tag as a bitflag.
+
+## Note
+
+This is the first procedural macro I've ever written, so I'm not sure if I'm doing it idiomatic or right.
+Feel free to open an issue or PR if you have any suggestions or improvements.