top-level reference of lib/licenses

Patch by eevans; reviewed by jbellis for CASSANDRA-371


git-svn-id: https://svn.apache.org/repos/asf/incubator/cassandra/branches/cassandra-0.4@810259 13f79535-47bb-0310-9956-ffa450edef68

LICENSE.txt

@@ -199,4 +199,9 @@
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
-   limitations under the License.
+   limitations under the License.
+
+
+NOTE: License texts and attribution for convenience copies of third-party
+libraries can be found in their respective LICENSE files in lib/licenses/.
+