Update documentation with information on seeds.

# Problem

The contribution documents should have instructions on how to handle seeds. Something along the lines of

# Suggested content

Make sure you update `priv/repo/seeds.exs` if your PR is doing anything to the schema, schema validations or anything that might be related.

When you submit your PR, run `mix ecto.reset`. This will drop and recreate the database, run all migrations and then run the seeds script. This command should always succeed.

# /end

The exact content should be discussed here.