Add explanation of what it means to mark an extension with a particular Inform build

[alice-blue]

Right now, when you upload an extension, you get a warning that you have not chosen any Inform builds, and people will not be able to access your extension from within the Inform IDE. I suggest listing the criteria for figuring out whether the extension should be marked with an Inform build or not. Does marking an Inform build imply that the extension follows the guidelines for extensions in the public library? That it's been tested? That the author plans to maintain and support it? That the author believes it is ready for public use? etc. It would also be helpful to have that sort of explanation available to the general public, even if they never upload any extensions to the site, so that they know why some extensions are marked with a build, and some are not.