And it is not just the GPL license file and source code repository that you need to add. You cannot remove or change any of our copyright notices, and you cannot represent our work as yours. Let me quote myself, to clear any misunderstanding:
https://nxtforum.org/nrs-releases/notice-to-nxt-clone-creators/The Nxt software is released as open source, which allows anyone to freely inspect the code, use it, modify it, and build on top of it.
No open source license however, neither the MIT License under which NRS versions prior to 1.5 were released, nor the GPL which applies to 1.5 and all later NRS versions, allows copying the work of others and presenting it as your own. Doing that is plain old plagiarism.
This also applies to release notes, change logs, and similar documentation included with the NRS distribution. You cannot copy, whether fully or partially, such texts, and make it appear as if you wrote them, or did the work described in them, while in fact it is the Nxt developers that did both.
Any derived work must make it clear which part of it, if any, is original, and give proper credit to the Nxt developers for their work.
You cannot remove the Nxt copyright notices, and you can only add your own copyright notice to a source code file if you really made a copyrightable contribution to this file. Changing the values of a few constants, or variable names, for example, is not a copyrightable contribution.
Starting from version 1.5, the NRS software is released under GNU General Public License (GPL), version 2. Including code from 1.5 or later Nxt version into your clone coin requires placing your work under GPLv2 too, with no exceptions. The Nxt core developers have the option of negotiating use of the Nxt code under a different license (a practice known as "selling exceptions to the GPL"), because they own the copyright. You will not have this option for your clone coin, once it incorporates GPL'd Nxt code, because you don't own the copyright to this Nxt code, and you are only allowed to use it under the conditions imposed by the GPL. Make sure you understand the long term implications of this, before "borrowing" Nxt code.
Here are some useful references about handling copyright in open source development, what is a copyrightable contribution, and compliance with the GPL:
https://softwarefreedom.org/resources/2012/ManagingCopyrightInformation.htmlhttps://softwarefreedom.org/resources/2007/originality-requirements.htmlhttps://softwarefreedom.org/resources/2007/gpl-non-gpl-collaboration.htmlhttps://softwarefreedom.org/resources/2014/SFLC-Guide_to_GPL_Compliance_2d_ed.htmlOh, and btw, it is not just the Nxt copyright you are violating, you have removed the copyright notices of all third-party software also included in the Nxt distribution - such as H2, Jetty, Lucene. Makes me wonder, why would a legitimate business do that?