better README

Author: brunjlar

README.md

@@ -1 +1,26 @@
 # stack2cabal
+
+This is a simple tool for using
+[`cabal new-build`](http://cabal.readthedocs.io/en/latest/nix-local-build-overview.html)
+in Haskell projects that have originally been designed
+for use with [`stack`](https://docs.haskellstack.org/en/stable/README/).
+
+The tool does two things:
+
+* `cabal new-build` (at the moment) handles only dependencies that are
+  either _local_ or _on Hackage_.
+  In `stack`, on the other hand, it is possible to use
+  dependencies from GitHub by specifying repository-URL and commit
+  in the `stack.yaml` file.
+  The `stack2cabal` tool scans the `stack.yaml` file for such dependencies,
+  clones the repositories into subfolder `./stack2cabal/`
+  and checks out the correct commit.
+
+* Using the packages listed in `stack.yaml` and the cloned git repositories,
+  `stack path --compiler-exe` to determine the correct GHC version
+  and `stack list-dependencies` to get the correct version of each dependency,
+  the `stack2cabal` tool creates the `cabal.project` file needed for
+  `cabal new-build`, 
+  thus making sure that `cabal new-build` will build the same packages as `stack`,
+  will use the same GHC version as `stack`
+  and will use the same dependency versions as `stack`.