Add simple README about our C bindings generation stuff

Author: TheBlueMatt

c-bindings-gen/README.md

@@ -0,0 +1,12 @@
+LDK C Bindings Generator
+========================
+
+This program parses a Rust AST from a single-lib.rs Rust crate (passed in on stdin) and generates a
+second crate which is C-callable (and carries appropriate annotations for cbindgen). It is usually
+invoked via the `genbindings.sh` script in the top-level directory.
+
+`genbindings.sh` requires that you have a rustc installed with the `wasm32-wasi` target available
+(eg via the `libstd-rust-dev-wasm32` package on Debian or `rustup target add wasm32-wasi` for those
+using rustup), cbindgen installed via `cargo install cbindgen` and in your `PATH`, and `clang`,
+`clang++`, `gcc`, and `g++` available in your `PATH`. It uses `valgrind` if it is available to test
+the generated bindings thoroughly for memory management issues.