ReadMe update

Update the ReadMe to explain the point of the project
Create VS 2010 based extension

This is a vanilla 2010 VSIX extension.  It was created with the project
wizard using the "Editor Margin" extension.  At this point the VSIX is
completely tied to Visual Studio 2010.  It can only be edited in and
deployed to that version.

Any attempt to open this project in 2012 or 2013 will be met with a
forced upgrade dialog.   Installing to either of these will also fail.

Over the next few edits this project will be updated such that it can be
edited in and deployed to any version of Visual Studio
Support VSIX deployment to any version of Visual Studio

The first change we will make is to enable the deployment of this
extension to any version of Visual Studio.  This is a relatively simple
process of updating the manifest to include the new versions of Visual
Studio in the SupprotedProducts section.

Once this change is added and the project rebuilt the resulting VSIX can
be installed on any version of Visual Studio

A quick note on Visual Studio versions

2010 = 10.0
2012 = 11.0
2013 = 12.0