MISC: update README and migration comment

models/migrations/v15.go

@@ -67,6 +67,8 @@ func generateAndMigrateGitHooks(x *xorm.Engine) (err error) {
 			wikiPath := repoBase + ".wiki.git"
 			log.Trace("[%04d]: %s", idx, repoPath)
 
+			// Note: we should not create hookDir here because update hook file should already exists inside this direcotry,
+			// if this directory does not exist, the current setup is not correct anyway.
 			hookDir := filepath.Join(repoPath, "hooks")
 			customHookDir := filepath.Join(repoPath, "custom_hooks")
 			wikiHookDir := filepath.Join(wikiPath, "hooks")