addMigrationinstead

class AddTitleToPosts < ActiveRecord::Migration
  def change
    add_column :posts, :title, :string
    add_column :posts, :image, :string
    add_column :posts, :link, :string
    add_column :posts, :documentation, :text
  end
  #NOTE: there's another way to write ^ a little more streamlined
  #For now do this for clarity
end

base for posts

class CreatePosts < ActiveRecord::Migration
  def change
    create_table :posts do |t|
      t.string :title
      t.string :image
      t.string :link
      t.text :documentation

      t.timestamps
    end
  end
end

So this is a base to work with. You can always add things later but that at least gets you started