Add license info for backgrounds

Signed-off-by: Marco <marcoambrosini@icloud.com>

img/backgrounds/COPYING.md

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+The following pictures located in this folder:
+
+1.jpg
+2.jpg
+3.jpg
+4.jpg
+5.jpg
+6.jpg
+
+Are licensed under the Unsplash license. More information
+at https://unsplash.com/license
+
+The images can be found at the following links
+https://unsplash.com/es/fotos/1hlFqUdFv1s
+https://unsplash.com/es/fotos/QMDap1TAu0g
+https://unsplash.com/es/fotos/NRQV-hBF10M
+https://unsplash.com/es/fotos/17NCG_wOkMY
+https://unsplash.com/es/fotos/qtRF_RxCAo0
+https://unsplash.com/es/fotos/KnLj3o9A66E