Add instruction to install Visual Studio SDK for ChromeDebug.

For C# projects, if a ProjectType GUID is not available, the vague error
"The project type is not supported by this installation" is given.
In this case the issue is usually that the Visual Studio SDK is not
installed.

R=zturner@chromium.org

Review URL: https://codereview.chromium.org/26460003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227550 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227550