Add a comment explaing rust_lib

src/rust_lib.rs

@@ -4,6 +4,15 @@ use std::path::PathBuf;
 
 use syn;
 
+/*
+ * Recursively parses a rust library starting at the root crate's directory.
+ *
+ * Inside a crate, `mod` and `extern crate` declarations are followed
+ * and parsed. To find an external crate, the parser looks in the
+ * parent directory of the root crate. This could be improved
+ * to read the crate's Cargo.toml to find the crate, but it works well
+ * enough to find the crates that matter for creating bindings.
+ */
 pub fn parse<F>(crate_dir: String,
                 items_callback: &F)
     where F: Fn(String, &Vec<syn::Item>)