Add note about data types to HACKING

Closes #1282

HACKING

@@ -214,6 +214,11 @@ Coding
   to will not be mutated within the function.
 * Don't let variable names shadow outer variables - use gcc's -Wshadow
   option.
+* Use the strictest possible data type where practical. For example
+  for an enumeration, use the actual enum type rather than just a
+  ``gint``, use a ``gchar`` for individual (ASCII/UTF-8) string
+  characters rather than ``gint``, and use a ``guint`` for integers
+  which cannot be negative rather than ``gint``.
 * Do not use G_LIKELY or G_UNLIKELY (except in critical loops). These
   add noise to the code with little real benefit.