Add C API (#5072)

sdk-c/README.md

@@ -0,0 +1,12 @@
+# Solana SDK-C
+
+This crate exposes a C API to Solana functions written in Rust. The crate generates a static library, and uses `cbindgen`
+to generate a header file during the build. To generate both:
+
+```shell
+$ cd <path/to/solana/repo>/sdk-c
+$ cargo build
+```
+
+This will generate the static library in `<path/to/solana/repo>/target/deps` and the header file in
+`<path/to/solana/repo>/sdk-c/include`.