tutorial: mention the WORKSPACE file

highlight that the way to obtain rules_haskell in tutorial/WORKSPACE is limited to the tutorial's case, otherwise better refer to the template created by start

also link to Common Haskell Build Use Cases to highlight the other mandatory thing in WORKSPACE file: obtaining GHC

I tried to make a minimal addition, so as to avoid polluting the tutorial; yet giving the sufficient pointers to make users able to set up a toolchain

Author: smelc

docs/haskell.rst

@@ -32,7 +32,7 @@ On a Unix system you will need the following tools installed.
 * ``libgmp``
 * ``libtinfo5``
 * ``make``
-* ``python3`` (``python`` also needs to be available in ``$PATH``. Depending on your distro, this might require installing the ``python`` meta-package, which might use Python 2 or 3, ``rules_haskell`` works with both.)
+* ``python3`` (``python`` also needs to be available in ``$PATH``. Depending on your distribution, this might require installing the ``python`` meta-package, which might use Python 2 or 3, ``rules_haskell`` works with both.)
 
 On Ubuntu you can obtain them by installing the following packages. ::
 
@@ -89,14 +89,32 @@ special:
   contains a ``BUILD.bazel`` file is a *package*. You will learn about
   packages later in this tutorial.)
 
-To designate a directory as a Bazel workspace, create an empty file
+To designate a directory as a Bazel workspace, create a file
 named ``WORKSPACE`` in that directory.
+This file defines `external dependencies`_.
 
 When Bazel builds the project, all inputs and dependencies must be in
 the same workspace. Files residing in different workspaces are
 independent of one another unless linked, which is beyond the scope of
 this tutorial.
 
+Understand the WORKSPACE file
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+File ``tutorial/WORKSPACE`` defines how to obtain ``rules_haskell``
+and a compiler toolchain.  Because this tutorial lives in ``rules_haskell``'s
+repository, ``rules_haskell`` is made available as follows::
+
+  local_repository(
+      name = "rules_haskell",
+      path = "..",
+  )
+
+We refer to the template created by the start_ script
+to make ``rules_haskell`` available outside a local clone.
+The ``WORKSPACE`` file also defines how to obtain the compiler toolchain,
+as detailed in section `picking a compiler`_.
+
 Understand the BUILD file
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -379,6 +397,8 @@ Happy building!
 .. _haskell_toolchain_library: http://api.haskell.build/haskell/haskell.html#haskell_toolchain_library
 .. _haskell_library: http://api.haskell.build/haskell/haskell.html#haskell_library
 .. _graphviz: https://www.graphviz.org/
+.. _start: https://rules-haskell.readthedocs.io/en/latest/haskell-use-cases.html#starting-a-new-project
+.. _picking a compiler: https://rules-haskell.readthedocs.io/en/latest/haskell-use-cases.html#picking-a-compiler
 .. _external dependencies: https://docs.bazel.build/versions/master/external.html
 .. _build encyclopedia: https://docs.bazel.build/versions/master/be/overview.html
 .. _C++ build tutorial: https://docs.bazel.build/versions/master/tutorial/cpp.html