Small READMEs for -derive and -core

sigma-compiler-core/README.md

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+`sigma-compiler-core` by Ian Goldberg, iang@uwaterloo.ca  
+Version 0.1.0-rc1, 2025-09-16
+
+This crate provides the core functionality for the macros in the
+[`sigma-compiler`](https://crates.io/crates/sigma-compiler) crate.

sigma-compiler-derive/README.md

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+`sigma-compiler-derive` by Ian Goldberg, iang@uwaterloo.ca  
+Version 0.1.0-rc1, 2025-09-16
+
+This crate provides the derive macros for the
+[`sigma-compiler`](https://crates.io/crates/sigma-compiler) crate.