f tweak c-bindings-gen readme with suggestions from val

Author: TheBlueMatt

c-bindings-gen/README.md

@@ -1,9 +1,11 @@
 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.
+This program parses a single-file-lib.rs Rust crate's AST 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