fixup readme to have more up to date cloning directions. Note that it doesnt layout how to clone from your own private tree
On Windows an extra step is needed.
Please also have a look at https://ghc.haskell.org/trac/ghc/ticket/8379.
I don't think this works out of the box. See: https://ghc.haskell.org/trac/ghc/wiki/Building/GettingTheSources#GettingaGHCrepositoryfromGitHub