docs: Update README to include seeding instructions (#1126)

Author: GabrielCWT

README.md

@@ -62,6 +62,12 @@ It is probably okay to use a different version of PostgreSQL or Erlang/OTP, but
    $ sudo service postgresql restart
    ```
 
+   By default, the database is populated with 10 students and 5 assessments. Each student will have a submission to the corresponding submission. This can be changed in `priv/repo/seeds.exs` with the variables `number_of_students`, `number_of_assessments` and `number_of_questions`. Save the changes and run:
+
+   ```bash
+   $ mix ecto.reset
+   ```
+
 4. Run the server on your local machine
 
    ```bash