add introductory comments to base64 examples

examples/base64.c

@@ -1,3 +1,13 @@
+// Example parser: Base64, syntax only.
+//
+// Demonstrates how to construct a Hammer parser that recognizes valid Base64
+// sequences.
+//
+// Note that no semantic evaluation of the sequence is performed, i.e. the
+// byte sequence being represented is not returned, or determined. See
+// base64_sem1.c and base64_sem2.c for examples how to attach appropriate
+// semantic actions to the grammar.
+
 #include "../src/hammer.h"
 
 const HParser* document = NULL;