Add a Doxyfile and preliminary module-level docs.

src/bip39.h

@@ -1,8 +1,17 @@
 #pragma once
 
+/**
+** \defgroup bip39 BIP39
+** Convert between raw bytes and \ref bip39_words.
+** See: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki.
+** @{
+*/
+
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
 
 bool tf_bip39_bytes_to_words(const uint8_t* bytes, size_t bytes_size, char* out_words, size_t words_size);
 bool tf_bip39_words_to_bytes(const char* words, uint8_t* out_bytes, size_t bytes_size);
+
+/** @} */