add NOTICE file and more detailed license info in main README

Author: herzbube

NOTICE

@@ -0,0 +1,15 @@
+libsgfc++
+Copyright 2020 Patrick Näf (herzbube@herzbube.ch)
+
+This product uses the source code provided by the SGFC command line utility to
+read, parse and write SGF data. A number of modifications have been made to the
+source code to be able to use it in a software library context. The changes are
+described extensively in doc/SgfcPatches.md. SGFC was developed by Arno Hollosi
+and has been released under the BSD license. The original software is available
+from
+  https://www.red-bean.com/sgf/sgfc/
+
+This product uses the test framework Catch2 for unit testing. Catch2 has been
+released under the Boost Software License 1.0. The original software is
+available from
+  https://github.com/catchorg/Catch2

README.md

@@ -68,7 +68,14 @@ The project build system is [documented here](doc/Build.md).
 
 ## License
 
-libsgfc++ is released under the [Apache License](http://www.apache.org/licenses/LICENSE-2.0) (2.0).
+libsgfc++ is released under the [Apache License](http://www.apache.org/licenses/LICENSE-2.0) (2.0). Here's the [link to the license file](LICENSE).
 
 SGFC is released under the terms of the [BSD License](https://opensource.org/licenses/BSD-3-Clause).
 
+- [Link to local license file](sgfc/COPYING). This works only if you have a local clone of the SGFC git submodule.
+- [Link to remote license file](https://bitbucket.org/arnoh/sgfc/src/master/COPYING). This link points to the `master` branch version of the license file.
+
+Catch2 is released under the [Boost Software License](https://www.boost.org/LICENSE_1_0.txt) (1.0).
+
+- [Link to local license file](test/Catch2/LICENSE.txt). This works only if you have a local clone of the SGFC git submodule.
+- [Link to remote license file](https://github.com/catchorg/Catch2/blob/master/LICENSE.txt). This link points to the `master` branch version of the license file.