README.md Add more project links

The operational wiki and JIRA links are added.  The Facebook and Twitter
connections are also included.
.gitignore excluding Microsoft ephemeral artifacts

The .gitignore is expanded to include exclusions that GitHub provides by
default when creating a git repository used for Windows development.
This annotated version includes some miscellaneous catches, such as
temporary files produced by a variety of editors.   In addition,
exclusion of Visual Studio Solution and Project files is added, with
explanatory comment.
DFAddDirContents: Don't use DFArray or DFError

Avoid using DFArray in DFAddDirContents, as a step towards removing the
dependency of the platform module upon core. We instead use the
DFDirEntryList type, which is simply a linked list of string values
which is manipulated manually, without requiring any convenience
functions defined in core.

This function also no longer takes a DFError object as a paramater,
though others in DFPlatform.h still do.