cmd/testscript: mention -continue flag in usage (#191)

The flag was added in #189 but not documented in the usage message.

cmd/testscript/help.go

@@ -14,7 +14,7 @@ The testscript command runs github.com/rogpeppe/go-internal/testscript scripts
 in a fresh temporary work directory tree.
 
 Usage:
-    testscript [-v] [-e VAR[=value]]... [-u] [-work] files...
+    testscript [-v] [-e VAR[=value]]... [-u] [-continue] [-work] files...
 
 The testscript command is designed to make it easy to create self-contained
 reproductions of command sequences.
@@ -42,6 +42,9 @@ succeed and the testscript file will be updated to reflect the actual content.
 As such, this is the cmd/testcript equivalent of
 testscript.Params.UpdateScripts.
 
+The -continue flag specifies that if an error occurs, the script will continue running.
+All errors will be printed and the exit status will be false.
+
 The -work flag prints the temporary work directory path before running each
 script, and does not remove that directory when testscript exits.