Update NEWS with base64 and base16 changes.

NEWS

@@ -46,6 +46,12 @@ NEWS for the Nettle 3.4 release
 
 	Miscellaneous:
 
+	* The base16 and base64 functions now use the type char * for
+	  ascii data, rather than uint8_t *. This eliminates the last
+	  pointer-signedness warnings when building Nettle. This is a
+	  minor API change, and applications may need to be adjusted,
+	  but the ABI is unaffected on all platforms I'm aware of.
+
 	* The contents of the header file nettle/version.h is now
 	  architecture independent, except in --enable-mini-gmp
 	  configurations.