Update documentation with information on seeds.

[begedin]

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.

[joshsmith]

@begedin what do you want feedback on here?

[begedin]

The actual content you'd want to put into the documents, as well as the place to put it. This is more of a "we should put something, somewhere" with the thing and the where not quite clear yet.