Added a License file

Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>

LICENSE.MIT

@@ -0,0 +1,13 @@
+Copyright Arash Partow (1999-2020)
+
+Free use of the Mathematical Expression Toolkit Library is
+permitted under the guidelines and in accordance with the 
+most current version of the MIT License.                  
+
+    http://www.opensource.org/licenses/MIT                    
+
+Unless required by applicable law or agreed to in writing, software
+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.