Add readthedocs and badge to README.md

README.md

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 Qubes core, version 3
 ---------------------
 
+<img src="https://readthedocs.org/projects/qubes-core-admin/badge/?version=latest">
+
 This is development branch of the Qubes OS core. This branch is subject to
 rebase without warning until further notice.
 
-API documentation is available: https://ftp.qubes-os.org/~woju/core3doc/.
+API documentation is available: https://qubes-core-admin.readthedocs.org/en/latest/.