Add Readme.md to the demo

docs/lmdb-d-demo/ReadMe.md

+This a demo program for the D language binding for lmDB
+
+The binding is directly derived from the official header files for lmdb.
+Some OO style code was ported from the lmdb++ header. See docs for
+original headers in the docs directory.
+
+A small demo application is provided in the docs directory.