general: update licensing and comply with OCCT LGPL2.1

README.md

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+# ARC
+
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+
+
+
+## License
+
+This project is licensed under the GNU Affero General Public License v3.0 - see [LICENSE.txt](LICENSE.txt) file for details.
+
+## Third-Party Dependencies
+
+This software uses **Open CASCADE Technology** (OCCT), which is licensed under the GNU LGPL version 2.1 with the following exception:
+
+### Open CASCADE Exception (version 1.0)
+> The object code (i.e. not a source) form of a "work that uses the Library"
+> can incorporate material from a header file that is part of the Library.
+> As a special exception to the GNU Lesser General Public License version 2.1,
+> you may distribute such object code incorporating material from header files
+> provided with the Open CASCADE Technology libraries (including code of CDL
+> generic classes) under terms of your choice, provided that you give
+> prominent notice in supporting documentation to this code that it makes use
+> of or is based on facilities provided by the Open CASCADE Technology software.
+
+### Source Code Availability
+The source code for Open CASCADE version 7.4.0 can be obtained from:
+- Repository: https://github.com/Open-Cascade-SAS/OCCT.git
+- Commit: `fd47711d682be943f0e0a13d1fb54911b0499c31`
+
+**This software makes use of facilities provided by Open CASCADE Technology.**