Add documentation on how use firestore.

firestore/README.md

@@ -4,7 +4,9 @@ A [Google Cloud Firestore](https://pkg.go.dev/cloud.google.com/go/firestore) bas
 
 ## Setup
 
-You should follow the instructions to [install and open a database](https://cloud.google.com/firestore/docs), and pass the database to `firestore.New()` to establish the session store.
+You should follow the instructions to [install and open a database](https://cloud.google.com/firestore/docs), and pass the database to `firestore.New()` to establish the session store. 
+
+The default collection is "Sessions". If you want to change that, store a custom CollectionRef in scsfs.Sessions.
 
 ## Example
 
@@ -12,22 +14,29 @@ You should follow the instructions to [install and open a database](https://clou
 package main
 
 import (
+	"context"
 	"io"
+	"log"
 	"net/http"
+	"os"
 
+	"cloud.google.com/go/firestore"
+	scsfs "github.com/alexedwards/scs/firestore"
 	"github.com/alexedwards/scs/v2"
-	"github.com/alexedwards/scs/firestore"
 )
 
 var sessionManager *scs.SessionManager
 
 func main() {
 	// Establish connection to Google Cloud Firestore.
-	// ...
-	
+	db, err := firestore.NewClient(context.Background(), os.Getenv("GOOGLE_CLOUD_PROJECT"))
+	if err != nil {
+		log.Fatal(err)
+	}
+
 	// Initialize a new session manager and configure it to use firestore as the session store.
 	sessionManager = scs.New()
-	sessionManager.Store = firestore.New(db)
+	sessionManager.Store = scsfs.New(db)
 
 	mux := http.NewServeMux()
 	mux.HandleFunc("/put", putHandler)
@@ -45,3 +54,17 @@ func getHandler(w http.ResponseWriter, r *http.Request) {
 	io.WriteString(w, msg)
 }
 ```
+
+The sample can be run by setting the environment variable GOOGLE_CLOUD_PROJECT, which should be proper project id. If you have the local firestore emulator (part of the Google Cloud SDK) installed you can start a local emulator and run the example like this:
+
+Start the emulator first:
+
+```sh
+gcloud beta emulators firestore start --host-port=localhost:8041
+```
+
+Then test the example program:
+
+```sh
+FIRESTORE_EMULATOR_HOST=localhost:8041 GOOGLE_CLOUD_PROJECT=test go run .
+```