Add a META.info file to allow Panda installation

The Panda installer tool requires a META.info file in the project's top
level directory to recognize it as installable into the Perl 6 library.
Add a ROADMAP with milestones before first release

This doc contains a first cut at the major milestones required before
cutting a first major release, as agreed by ssutch and japhb.
Add a README.md

This version is relatively short but covers all the bases for now,
including linking to the ROADMAP doc.  Using Markdown format to be
more friendly to GitHub users.
Stub in an overview doc

This stub just points users to the official Google documentation,
especially the Developer Guide and FAQ, noting that the actual Perl 6
API docs will be here of course.