RELEASE: Add info about Softonic

docs/RELEASE.txt

@@ -1,5 +1,5 @@
 Release checklist for Tux Paint
-Last updated 2023-03-16 -bjk
+Last updated 2023-05-01 -bjk
 
 During development
 ------------------
@@ -186,6 +186,15 @@ Announcements
      - Site: https://www.macupdate.com/
      - Submissions: [currently managed by Bill K.]
 
+   + Softonic
+     - Site: https://en.softonic.com/
+     - Entries:
+       + https://tux-paint.en.softonic.com/windows
+       + https://tux-paint.en.softonic.com/mac
+       + https://tux-paint.en.softonic.com/android
+     - Submissions: https://hello.softonic.com/faqs/
+       (links to a Google Form under "Softonic Developer Hub shutdown")
+
  * Linux repositories
 
    + Fedora -- process TBD