add a README

Author: nikomatsakis

Cargo.lock

@@ -1,11 +1,12 @@
 [package]
 name = "formality-core"
-version = "0.1.0"
+version = "0.1.1"
 edition = "2021"
 license = "MIT OR Apache-2.0"
 description = "Language-independent formality system used by a-mir-formality"
 homepage = "https://rust-lang.github.io/a-mir-formality/"
 repository = "https://github.com/rust-lang/a-mir-formality/"
+readme = "README.md"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 

crates/formality-core/Cargo.toml

@@ -0,0 +1,4 @@
+# formality-core
+
+`formality_core` is the language independent part of a-mir-formality.
+It can be reused by other projects looking to model formal semantics of languages besides Rust.