Add README.md with notes on branches.

The extra-doc-files property requires cabal-version >= 1.18 (c.f.
http://coldwa.st/e/blog/2013-08-21-Cabal-1-18.html).  Bump this requirement
from 1.10.
Update cabal file to reflect renamed repository to syntax-trees.git.

NB: Only the name of the repository is changed.
Bump version to 0.1.2.4 from 0.1.2.3.