add LICENSE, fixes flatironinstitute#115

Author: ahbarnett

LICENSE

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+Copyright (C) 2018-2021 The Simons Foundation, Inc. - All Rights Reserved.
+
+Lead developer: Yu-Hsuan Melody Shih (New York University).
+
+Other developers: (see github site for full list)
+
+Garrett Wright (Princeton)
+Joakim Anden (KTH)
+Johannes Blaschke (LBNL)
+Alex Barnett (CCM, Flatiron Institute)
+
+This project came out of Melody's 2018 and 2019 summer internships at
+the Flatiron Institute, advised by Alex Barnett.
+
+------
+
+cuFINUFFT is licensed under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance with the
+License.  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+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.
+
+------
+
+Certain parts of this repository are contributed by others.
+For their license info, see:
+
+contrib/legendre_rule_fast.license
+fortran/cmcl_license.txt
+
+------
+
+If you find this library useful, or it helps you in creating software
+or publications, please let us know, and acknowledge that fact by citing our
+repository:
+
+  https://github.com/flatironinstitute/cufinufft
+
+and the publication:
+
+cuFINUFFT: a load-balanced GPU library for general-purpose nonuniform FFTs, Yu-hsuan Shih, Garrett Wright, Joakim Andén, Johannes Blaschke, Alex H. Barnett. PDSEC2021 conference (best paper prize). https://arxiv.org/abs/2102.08463

contrib/legendre_rule_fast.license

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+LICENSE info for legendre_rule_fast.c ONLY:
+
+According to
+https://people.sc.fsu.edu/~jburkardt/c_src/legendre_rule_fast/legendre_rule_fast.html
+
+The computer code and data files described and made available on this web page are distributed under the GNU LGPL license:
+
+https://www.gnu.org/licenses/lgpl-3.0.en.html