Set some example ENV defaults to help get set up / started more easily (#341)

When setting up and running the project just now, I found I had to set
these values in my `.env` file. It'd be easier for anyone in future to
just copy them across from the `.env.example` file.

Author: grega

.env.example

@@ -1,4 +1,4 @@
-ALLOWED_ORIGINS=localhost:3002,localhost:3010,editor.rpfdev.com,localhost:3009
+ALLOWED_ORIGINS=localhost:3000,localhost:3002,localhost:3010,editor.rpfdev.com,localhost:3009
 
 AWS_ACCESS_KEY_ID=changeme
 AWS_S3_ACTIVE_STORAGE_BUCKET=changeme
@@ -7,9 +7,10 @@ AWS_SECRET_ACCESS_KEY=changeme
 
 GITHUB_WEBHOOK_SECRET=test_token
 
-POSTGRES_HOST=changeme
-POSTGRES_USER=changeme
-POSTGRES_PASSWORD=changeme
+POSTGRES_DB=development
+POSTGRES_HOST=db
+POSTGRES_PASSWORD=password
+POSTGRES_USER=postgres
 
 USERINFO_API_URL=http://localhost:6000
 USERINFO_API_KEY=1234