[day-04] add training schema

With this we can represent a traning program for a user.

priv/repo/migrations/20210624003633_create_trainings_table.exs

@@ -0,0 +1,13 @@
+defmodule Wabanex.Repo.Migrations.CreateTrainingsTable do
+  use Ecto.Migration
+
+  def change do
+    create table(:trainings) do
+      add :start_date, :date, null: false
+      add :end_date, :date
+      add :user_id, references(:users), null: false
+
+      timestamps()
+    end
+  end
+end