add user and db creation section

guides/ubuntu/pages/mongodb-install.md

@@ -152,6 +152,42 @@ security:
 
 7. Save the file and exit the editor. You can do this by pressing `CTRL + X`, then `Y`, then `ENTER`.
 
+## Adding a user / database
+
+1. Connect to MongoDB, so that we can create a user. Run the following command:
+
+```bash
+mongo -u admin -p --authenticationDatabase admin
+```
+
+This will ask you for the password that you set earlier in the Securing MongoDB section.
+
+2. Switch to the database that you want to add a user to eg: `forum`. Run the following command:
+
+```bash
+use forum
+```
+
+3. Create a user. Make sure to change the `db` to match the database used above. Run the following command:
+
+```bash
+db.createUser(
+  {
+    user: "forum",
+    pwd:  "SET ME", # Generate a random string with: openssl rand -base64 32 | cut -c1-32
+    roles: [ { role: "readWrite", db: "forum" } ]
+  }
+)
+```
+
+4. Exit the mongo shell. Run the following command:
+
+```bash
+exit
+```
+
+You have now created a user for the `forum` database. You can repeat this process for any other databases that you want to create users for.
+
 ## Optional: Allowing remote connections
 
 1. Open the MongoDB configuration file. Run the following command: