EGM2008: add reference to about activity.

src/main/res/values/do_not_translate.xml

@@ -17,6 +17,11 @@ limitations under the License.
     <string name="app_web_url" translatable="false">https://github.com/OpenTracksApp/OpenTracks</string>
     <string name="app_name" translatable="false">OpenTracks</string>
 
+    <string name="third_party_data" translatable="false">
+        <a href="https://earth-info.nga.mil/">National Geospatial-Intelligence Agency (NGA)\'s EGM2008 model</a> 5\' minute resolution.
+        Data provided by <a href="https://geographiclib.sourceforge.io/html/geoid.html">GeographicLib</a>.
+    </string>
+
     <!-- List here all used libraries (will be shown in the about activity -->
     <string name="third_party_libraries" translatable="false">
         <a href="https://developer.android.com/topic/libraries/support-library/">Google\'s androidX support</a> (Apache License, Version 2.0) \n

src/main/res/values/strings.xml

@@ -20,6 +20,7 @@ limitations under the License.
     <string name="about_version_name">Version name: %1$s</string>
     <string name="about_version_code">Version code: %1$d</string>
     <string name="about_history">OpenTracks is a fork of <a href="https://code.google.com/archive/p/mytracks/"><i>Google\'s MyTracks</i></a> (discontinued in 2016).</string>
+    <string name="about_third_party_data">Third-party data</string>
     <string name="about_libraries">Libraries</string>
     <!-- Help -->
     <string name="help_feature_title">OpenTracks has the following features:</string>