Add testable example in README

This adds a testable example for the README to be able to make the user
experience for new users better.

examples/sessions.go

@@ -0,0 +1,42 @@
+package examples
+
+import (
+	"log"
+	"net/http"
+	"time"
+
+	"github.com/antonlindstrom/pgstore"
+)
+
+// ExampleHandler is an example that displays the usage of PGStore.
+func ExampleHandler(w http.ResponseWriter, r *http.Request) {
+	// Fetch new store.
+	store, err := pgstore.NewPGStore("postgres://user:password@127.0.0.1:5432/database?sslmode=verify-full", []byte("secret-key"))
+	if err != nil {
+		log.Fatalf(err.Error())
+	}
+	defer store.Close()
+
+	// Run a background goroutine to clean up expired sessions from the database.
+	defer store.StopCleanup(store.Cleanup(time.Minute * 5))
+
+	// Get a session.
+	session, err := store.Get(r, "session-key")
+	if err != nil {
+		log.Fatalf(err.Error())
+	}
+
+	// Add a value.
+	session.Values["foo"] = "bar"
+
+	// Save.
+	if err = session.Save(r, w); err != nil {
+		log.Fatalf("Error saving session: %v", err)
+	}
+
+	// Delete session.
+	session.Options.MaxAge = -1
+	if err = session.Save(r, w); err != nil {
+		log.Fatalf("Error saving session: %v", err)
+	}
+}