add link to kwant source downloads page on the 'install' page

content/install.rst

@@ -327,5 +327,6 @@ Building from source
 ====================
 If no packages are available for the system you use, or if you would like to
 build Kwant from source for another reason (expert users may want to customize
-Kwant to use certain optimized versions of libraries), please consult the
+Kwant to use certain optimized versions of libraries), you can `download
+the source code <https://downloads.kwant-project.org/kwant>`_ and consult the
 documentation on `how to install Kwant from source <doc/1/pre/install.html>`_.