[day-04] add exercise schema

So a training is a collection of exercises that have a description and a
protocol to be followed.

priv/repo/migrations/20210624003645_create_exercises_table.exs

@@ -0,0 +1,15 @@
+defmodule Wabanex.Repo.Migrations.CreateExercisesTable do
+  use Ecto.Migration
+
+  def change do
+    create table(:exercises) do
+      add :name, :string, null: false
+      add :video_url, :string
+      add :protocol_description, :string, null: false
+      add :repetitions, :string, null: false
+      add :training_id, references(:trainings), null: false
+
+      timestamps()
+    end
+  end
+end