Creating 'archived' branch for GitHub

Since we don't want to break all links to our 'old' GitHub
repository, we'll 'archive' it instead by switching the main branch
to an empty one, with just a README.


git-svn-id: https://svn.apache.org/repos/asf/sling/branches/archived@1812849 13f79535-47bb-0310-9956-ffa450edef68
tree: 4b825dc642cb6eb9a060e54bf8d69288fbee4904

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