Add MIT License (Drain3) (#25)

Add license notice for MIT code from Drain3 algorithm.

LICENSE

@@ -199,3 +199,19 @@
    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.
+
+=======================================================================
+AIOps Engine for SkyWalking Subcomponents:
+
+The AIOps Engine for SkyWalking project contains code with 
+separate copyright notices and license terms. 
+Your use of the source code for the these subcomponents is subject to 
+the terms and conditions of the following licenses.
+
+========================================================================
+MIT licenses
+========================================================================
+
+The following components are provided under the MIT License. See project link for details.
+
+   Drain machine learning model files from IBM/Drain3: https://github.com/IBM/Drain3 MIT