Add documentation for fetching submodules.

lib/spack/docs/packaging_guide.rst

@@ -604,6 +604,7 @@ Git fetching is enabled with the following parameters to ``version``:
   * ``tag``: name of a tag to fetch.
   * ``branch``: name of a branch to fetch.
   * ``commit``: SHA hash (or prefix) of a commit to fetch.
+  * ``submodules``: Also fetch submodules when checking out this repository.
 
 Only one of ``tag``, ``branch``, or ``commit`` can be used at a time.
 
@@ -660,6 +661,17 @@ Commits
   could just use the abbreviated commit hash.  It's up to the package
   author to decide what makes the most sense.
 
+Submodules
+
+  You can supply ``submodules=True`` to cause Spack to fetch submodules
+  along with the repository at fetch time.
+
+  .. code-block:: python
+
+     version('1.0.1', git='https://github.com/example-project/example.git',
+             tag='v1.0.1', submdoules=True)
+
+
 Installing
 ^^^^^^^^^^^^^^