Add a Doxyfile and preliminary module-level docs.

src/bcrypt.js.h

@@ -1,5 +1,14 @@
 #pragma once
 
+/**
+** \defgroup bcrypt_js bCrypt
+** Exposes bcrypt to script, where it is used for hashing and verifying
+** passwords.
+** @{
+*/
+
 typedef struct JSContext JSContext;
 
 void tf_bcrypt_register(JSContext* context);
+
+/** @} */