gui: add README.md and LICENSE.TXT for icons (#5387)

* gui: add README.md and LICENSE.TXT for icons

This PR adds a `README.md` containing author @Cracert, license and source of icon files.

Also adding `LICENSE.TXT` as required by icon license.

* use ASCII version of LICENSE.TXT

Author: neteler

gui/icons/LICENSE.TXT

@@ -0,0 +1,28 @@
+LICENSE
+
+Original GIS icons theme was created by Robert Szczepanek [1] and is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License [2].
+Fill free to use it for GIS software or for any other purposes. I only ask you to let me know about that and to include licence.txt file in your work.
+
+TITLE:			gis-0.1
+DESCRIPTION:	GIS icon theme
+AUTHOR:			Robert Szczepanek
+CONTACT:		robert at szczepanek pl
+SITE:			http://robert.szczepanek.pl/
+
+
+ACKNOWLEDGEMENTS
+
+OSGeo community [3] helped in whole design process.
+Especially I want to acknowledge GRASS and QGIS team members for creative comments and support:
+Martin Landa
+Michael Barton
+Tim Sutton
+Borys Jurgiel
+
+Robert Szczepanek
+Cracow 2009
+Poland
+
+[1] http://robert.szczepanek.pl/
+[2] http://creativecommons.org/licenses/by-sa/3.0/
+[3] http://www.osgeo.org/

gui/icons/README.md

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+## GIS icons theme mainly designed for GIS applications
+
+### Developed by
+
+Robert Szczepanek <robert.szczepanek AT uj.edu.pl>
+
+### Source
+
+<https://github.com/Cracert/GIS-icons>
+
+### License
+
+GIS icons by Robert Szczepanek is licensed under a
+Creative Commons Attribution-Share Alike 3.0 Unported License.
+Feel free to use it in any GIS software or for other purposes.
+I only ask you to let me know about that and to include
+LICENSE.TXT file in your work.