Some docs

runtime/src/lib.rs

@@ -14,6 +14,8 @@
 // You should have received a copy of the GNU General Public License
 // along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
 
+///! The Cumulus runtime to make a runtime a parachain.
+
 #![cfg_attr(not(feature = "std"), no_std)]
 
 use rstd::vec::Vec;