DigitalWriteFast: added README about license and author

libraries/DigitalWriteFast/README.txt

@@ -0,0 +1,10 @@
+This implementation of DigitalWriteFast() and related functions is
+based on the following code:
+
+  http://code.google.com/p/digitalwritefast/
+
+This code is available under the GNU General Public License version 3,
+and was originally written by johnraines24@gmail.com
+
+The code was imported into APM by Randy Mackay and Andrew Tridgell
+