Add missing line in license (#36)

license.txt

@@ -31,4 +31,12 @@ CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
 
-
+You are under no obligation whatsoever to provide any bug fixes, patches, or
+upgrades to the features, functionality or performance of the source code
+("Enhancements") to anyone; however, if you choose to make your Enhancements
+available either publicly, or directly to Lawrence Berkeley National Laboratory,
+without imposing a separate written license agreement for such Enhancements,
+then you hereby grant the following license: a non-exclusive, royalty-free
+perpetual license to install, use, modify, prepare derivative works, incorporate
+into other computer software, distribute, and sublicense such enhancements or
+derivative works thereof, in binary and source code form.