doc, add pdd to list of similar projects

Author: hroptatyr

README.md

@@ -409,6 +409,7 @@ In no particular order and without any claim to completeness:
 + dateexpr: <http://www.eskimo.com/~scs/src/#dateexpr>
 + allanfalloon's dateutils: <https://github.com/alanfalloon/dateutils>
 + yest <http://yest.sourceforge.net/>
++ pdd <https://github.com/jarun/pdd>
 
 Use the one that best fits your purpose.  And in case you happen to like
 mine, vote: [dateutils' openhub page](https://www.openhub.net/p/dateutils)