update installation instructions

content/install.rst

@@ -122,6 +122,15 @@ You can then tell ``pip`` to use this ``build.conf`` when installing kwant::
 
     sudo pip install --global-option="--configfile=/path/to/build.conf" kwant
 
+
+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
+documentation on `how to install Kwant from source <doc/1/pre/install.html>`_.
+
+
 Ubuntu and derivatives
 ======================