darcs patch: add README to warn about autoreconf

[Chris Smith]

Ian Lynagh wrote:
> This is really up to Christopher as the regex-posix maintainer, but we
> should probably try to be consistent between Cabal packages here, i.e.
> this sort of thing ought to be in the same place in any Cabal darcs
> checkout.
>
> I don't think README is the best name, as some Cabal packages will
> want to put README into the source tarball. Perhaps README.darcs?

I copied the text from identically named README files in other packages.  If 
you do it differently here, they'll need to be changed as well.

-- 
Chris Smith