Define "free software" (#591)

app/src/main/res/values/strings.xml

@@ -236,7 +236,12 @@
     <string name="about_tusky_license">Tusky is free and open-source software.
         It is licensed under the GNU General Public License Version 3.
         You can view the license here: https://www.gnu.org/licenses/gpl-3.0.en.html</string>
-    <!-- note to translators: the url can be changed to link to the localized version of the license -->
+    <!-- note to translators:
+         * you should think of “free” as in “free speech,” not as in “free beer”.
+           We sometimes call it “libre software,” borrowing the French or Spanish word for “free” as in freedom,
+           to show we do not mean the software is gratis. Source: https://www.gnu.org/philosophy/free-sw.html
+         * the url can be changed to link to the localized version of the license.
+    -->
     <string name="about_project_site">
         Project website:\n
         https://tuskyapp.github.io