add License NOTICE

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

NOTICE

@@ -0,0 +1,12 @@
+Additional terms under GNU AGPL version 3 section 7.3(b) and 13.1:
+
+If you have included SFTPGo so that it is offered through any network
+interactions, including by means of an external user interface, or
+any other integration, even without modifying its source code and then
+SFTPGo is partially, fully or optionally configured via your frontend,
+you must provide reasonable but clear attribution to the SFTPGo project
+and its author(s), not imply any endorsement by or affiliation with the
+SFTPGo project, and you must prominently offer all users interacting
+with it remotely through a computer network an opportunity to receive
+the Corresponding Source of the SFTPGo version you include by providing
+a link to the Corresponding Source in the SFTPGo source code repository.

README.md

@@ -79,7 +79,7 @@ Thank you to [Incode](https://www.incode.it/) for helping us to improve the UI/U
 
 ## License
 
-SFTPGo source code is licensed under the GNU AGPL-3.0-only.
+SFTPGo source code is licensed under the GNU AGPL-3.0-only with [additional terms](./NOTICE).
 
 The [theme](https://keenthemes.com/products/templates-mega-bundle) used in WebAdmin and WebClient user interfaces is proprietary, this means: