Add Wiki theme upgrade strategy

[manuelvsousa]

• Add fancy badges to README
• Add instructions about the theme upgrade strategy to README
• Add theme as subtree and migrates all custom modifications to it

[terjanq]

I haven't worked with submodules so I CC @empijei for this change.

[empijei]

Maybe it would be better to fork the theme under xsleaks/ and just change the submodule here?

I know I suggested smth different in the past but this commit completely loses the history and origin of the theme and just blobs together a quite large change.