Revert "remove mention of -s flag from README as it shouldn't need to…

… be used anymore"

This reverts commit 906b488.

unfortunately there is still a short period between -beta and the next
release where we still need to force snapshots

README.md

@@ -25,9 +25,13 @@ up to date
 ### Check for upgrades using index.txt:
 `obsdpkgup -n`
 
+### Force checking snapshot directory for upgrades:
+`obsdpkgup -s`
+
 ### Run and apply found package upgrades:
 `obsdpkgup |doas sh`
 
+
 ### Cron mode (don't output anything when packages are up-to-date):
 `obsdpkgup -c`