Convert README file to markdown.

Also change 'darcs' to 'git'.

README → Readme.md

-To build this package using Cabal directly from darcs, you must run
+# time
+
+This is the haskell time library that is bundled with [GHC][GHC] the Glasgow/
+Glorious Haskell compiler.
+
+To build this package using Cabal directly from git, you must run
 "autoreconf" before the usual Cabal build steps (configure/build/install).
 autoreconf is included in the GNU autoconf tools.  There is no need to run
 the "configure" script: the "setup configure" step will do this for you.
+
+[GHC]: https://www.haskell.org/ghc/