Add simple README about our C bindings generation stuff

diff --git a/c-bindings-gen/README.md b/c-bindings-gen/README.md
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+LDK C Bindings Generator
+========================
+
+This program parses a Rust crate's AST from a single lib.rs 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, which converts the lightning
+crate into a single file with a call to
+`RUSTC_BOOTSTRAP=1 cargo rustc --profile=check -- -Zunstable-options --pretty=expanded`.
+
+`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.