Add documentation for date-button.

date-button/README.md

+Date Inserter
+=============
+Simple scripts which inserts the current date-time when a given button
+is pressed.
+
+When installed a button will appear in the top left corner, pressing
+it prompts you to enter a key sequence. Later, whenever that key
+sequence is entered the current date-time is entered into the current
+text field.