resources : added docs fixes #1551 (#1627)

lib/spack/docs/packaging_guide.rst

@@ -759,6 +759,26 @@ Fetching a revision
 Subversion branches are handled as part of the directory structure, so
 you can check out a branch or tag by changing the ``url``.
 
+Expanding additional resources in the source tree
+-------------------------------------------------
+
+Some packages (most notably compilers) provide optional features if additional
+resources are expanded within their source tree before building. In Spack it is
+possible to describe such a need with the ``resource`` directive :
+
+  .. code-block:: python
+
+     resource(
+        name='cargo',
+        git='https://github.com/rust-lang/cargo.git',
+        tag='0.10.0',
+        destination='cargo'
+     )
+
+Based on the keywords present among the arguments the appropriate ``FetchStrategy``
+will be used for the resource. The keyword ``destination`` is relative to the source
+root of the package and should point to where the resource is to be expanded.
+
 Automatic caching of files fetched during installation
 ------------------------------------------------------