Add description and README

savvy-ffi/Cargo.toml

@@ -1,5 +1,6 @@
 [package]
 name = "savvy-ffi"
+description = "Minimal FFI bindings for R's C API"
 version.workspace = true
 edition.workspace = true
 authors.workspace = true

savvy-ffi/README.md

@@ -0,0 +1,16 @@
+# savvy-ffi
+
+Minimal FFI bindings for R's C API. This contains only a subset of APIs
+sufficient for savvy framework. If you are looking for more complete one,
+[libR-sys](https://crates.io/crates/libR-sys) is probably what you want.
+
+Some more notable differences between libR-sys are:
+
+* This is NOT a sys crate. Savvy-ffi is intended to be used within an R package,
+  which compiles a staticlib from Rust code first and then links it to R. At the
+  point of compilation by cargo, savvy-ffi is not yet linked, so this is fine.
+
+* All definitions are written by hand, with some help of bindgen, into a single
+  file. There's no automatic version switch or platform switch. If some switch
+  is needed, it will be provided as a feature (e.g. `r_4_4_0`) and it's user's
+  responsibility to set it properly.