More info in the fallbackdir changes file

changes/ticket40061

-  o Major feature (fallbackdir):
-    - Rebuilding list for 2020. This includes 144 relays as fallback
-      directories. Closes ticket 40061.
-
+  o Major feature (fallback directory list):
+    - Replace the 148 fallback directories originally included in
+      Tor 0.4.1.4-rc (of which around 105 are still functional) with
+      a list of 144 fallbacks generated in July 2020.
+      Closes ticket 40061.