clarify usage text, add help flags

Also remove the -toolexec equivalent, as it's becoming longer now that
we have GARBLE_DIR, and it might become out of date in the future again.
We don't want users to assume it will work forever.

README.md

@@ -6,9 +6,7 @@ Obfuscate a Go build. Requires Go 1.13 or later.
 
 	garble build [build flags] [packages]
 
-which is equivalent to the longer:
-
-	GARBLE_DIR="$PWD" go build -a -trimpath -toolexec=garble [build flags] [packages]
+See `garble -h` for up to date usage information.
 
 ### Purpose
 

main.go

@@ -38,9 +38,13 @@ Usage of garble:
 
 	garble build [build flags] [packages]
 
-which is equivalent to the longer:
+The tool supports wrapping the following Go commands - run "garble cmd [args]"
+instead of "go cmd [args]" to add obfuscation:
 
-	go build -a -trimpath -toolexec=garble [build flags] [packages]
+	build
+	test
+
+garble does not have flags of its own at this moment.
 `[1:])
 	flagSet.PrintDefaults()
 	os.Exit(2)
@@ -144,7 +148,13 @@ func mainErr(args []string) error {
 
 	// If we recognise an argument, we're not running within -toolexec.
 	switch cmd := args[0]; cmd {
+	case "help":
+		flagSet.Usage()
 	case "build", "test":
+		switch args[1] {
+		case "-h", "-help", "--help":
+			flagSet.Usage()
+		}
 		wd, err := os.Getwd()
 		if err != nil {
 			return err

testdata/scripts/help.txt

@@ -1,14 +1,24 @@
 ! garble
-stderr -count=1 'Usage of'
-stderr '\tgo build'
+stderr -count=1 'Usage of garble'
+stderr 'garble cmd'
+! stderr 'usage: go build'
 ! stdout .
 
 ! garble -h
-stderr 'Usage of'
+stderr 'Usage of garble'
+! stdout .
+
+! garble help
+stderr 'Usage of garble'
+! stdout .
+
+! garble build -h
+stderr 'Usage of garble'
+! stderr 'usage: go build'
 ! stdout .
 
 ! garble -badflag
-stderr 'Usage of'
+stderr 'Usage of garble'
 ! stdout .
 
 ! garble badcmd