Add a follow model

migrations/2018-05-01-124607_create_follow/down.sql

@@ -0,0 +1,2 @@
+-- This file should undo anything in `up.sql`
+DROP TABLE follows;

migrations/2018-05-01-124607_create_follow/up.sql

@@ -0,0 +1,6 @@
+-- Your SQL goes here
+CREATE TABLE follows (
+    id SERIAL PRIMARY KEY,
+    follower_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
+    following_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL
+)