Add a dummy file for "Docs/manual.chm" as we need it for "make dist" to work.

Docs/manual.chm:
  BitKeeper file /MySQL/M50/push-5.0/Docs/manual.chm

Docs/manual.chm

+
+*********************************************************
+
+This is a dummy placeholder file for "manual.chm" in the
+MySQL source trees.
+
+Note, that the documentation has been moved into a separate
+BitKeeper source tree named "mysqldoc" - do not attempt to edit this
+file! All changes to it should be done in the mysqldoc tree.
+
+This dummy file is being replaced with the actual file from the
+mysqldoc tree when building the official source distribution.
+
+*********************************************************