Add Modification Notice to Readme

As required under the LGPL, a prominent notice has been added to the
main readme file to inform the user that this is a modified version
of ROOT.

README.md

 <img src="https://root-forum.cern.ch/uploads/default/original/2X/3/3fb82b650635bc6d61461f3c47f41786afad4548.png" align="right"  height="50"/>
 
+## :warning: Modified Version :warning:
+
+This version of [ROOT](https://root.cern) has been modified to be included in the [Rutherford Experiment Online Lab Course at Ruhr-Universität Bochum](https://gitlab.ep1.rub.de/fp311online).
+The original source can be found at GitHub: https://github.com/root-project/root
+
+This modified version is hosted at: https://gitlab.ep1.rub.de/fp311online/cern-root
+
+### Purpose
+
+The main distribution path of the aforementioned software under Linux is Flatpak because it is distribution-agnostic.
+Unfortunately, the official ROOT version does not work properly inside the sandbox created by Flatpak due to the special needs of Cling, the runtime C++ parser of ROOT.
+These modifications do allow using ROOT inside the sanbox, which is their purpose.
+
+### Changelog
+
+* Added the ` __ROOFIT_NOBANNER` compiler flag to `libRooFitCore`
+* Added build option `BUILD_FOR_FLATPAK`
+* Adapted Cling C++ header search paths for use inside the Flatpak sandbox if `BUILD_FOR_FLATPAK` is enabled
+
+### License
+
+This version of ROOT including the modifications is licensed under the [GNU Lesser General Public Licensei (LGPL), version 2.1 or later](ihttps://www.gnu.org/licenses/old-licenses/lgpl-2.1.html).
+
+The Rutherford Experiment Online Lab Course and this version of ROOT form a combined work when they are distributed in one Flatpak bundle. This bundle is licensed under the [GNU General Public Licensei (GPL), version 3 or later](https://www.gnu.org/licenses/gpl-3.0.html) under the “upgrade clauses“ of the LGPL.
+
+### Contact Information
+
+The author of this modified version can be contacted as follows:
+
+Dr. Tobias Triffterer
+
+Ruhr-Universität Bochum  
+Institut für Experimentalphysik I  
+NB 2/126 – Fach-Nr. 125  
+Universitätsstraße 150  
+44801 Bochum  
+Germany  
+
+E-Mail: tobias@ep1.ruhr-uni-bochum.de  
+[S/MIME Certificate](https://ep1.ruhr-uni-bochum.de/~tobias/tobias_ep1.pem) (Fingerprint: DF35 4C01 A72C E689 2D95 F52D 8426 5187 BE21 2DEE)  
+[GnuPG/OpenPGP Key](https://ep1.ruhr-uni-bochum.de/~tobias/tobias_ep1.asc) (Fingerprint: 32FC 7266 0504 437B BF8A AFB6 7B74 849A 85A6 0E50)  
+
+EP1 GitLab Username: @tobias  
+Matrix-ID: [\@trifftmp:ruhr-uni-bochum.de](https://matrix.to/#/@trifftmp:ruhr-uni-bochum.de)
+
+---
+
+Original Readme:
+
 ## About
 
 The ROOT system provides a set of OO frameworks with all the functionality