Extend help output with short overview

The list of subcommands can be confusing for first time users. This
adds a short overview of the most important commands to help get people
started.

changelog/unreleased/pull-3225

@@ -0,0 +1,6 @@
+Enhancement: Extend cli help output
+
+Added a short overview of the most important commands to the commandline
+help output to help new users get started.
+
+https://github.com/restic/restic/pull/3225

cmd/restic/main.go

@@ -18,14 +18,40 @@ import (
 	"github.com/restic/restic/internal/errors"
 )
 
+// LongDescription is shown in the cli help output
+const LongDescription = `
+restic is a backup program which allows saving multiple revisions of files and
+directories in an encrypted repository stored on different backends.
+
+To get started with a local test repository, first define some environment variables:
+    export RESTIC_REPOSITORY=/tmp/restic-example
+    export RESTIC_PASSWORD=some-strong-password
+
+Initialize the repository (first time only):
+    restic init
+
+Create your first backup:
+    restic backup /etc/hosts 
+
+You can list all the snapshots you created with:
+    restic snapshots
+
+You can restore a backup by noting the snapshot ID you want and running:
+    restic restore --target /tmp/some-target-dir your-snapshot-ID
+
+It is a good idea to periodically check your repository's metadata:
+    restic check
+    # or full data:
+    restic check --read-data
+
+The full documentation can be found at https://restic.readthedocs.io/
+`
+
 // cmdRoot is the base command when no other command has been specified.
 var cmdRoot = &cobra.Command{
 	Use:               "restic",
 	Short:             "Backup and restore files",
-	Long: `
-restic is a backup program which allows saving multiple revisions of files and
-directories in an encrypted repository stored on different backends.
-`,
+	Long:              LongDescription,
 	SilenceErrors:     true,
 	SilenceUsage:      true,
 	DisableAutoGenTag: true,