Add Show Pending Migrations to rails til.

rails/show-pending-migrations.md

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+# Show Pending Migrations
+
+Rails comes with a built-in rake task that allows you to check the status
+of migrations in the project.
+
+```bash
+$ rake db:migrate:status
+
+database: pokemon_development
+
+ Status   Migration ID    Migration Name
+--------------------------------------------------
+   up     20150219143706  Create pokemon table
+  down    20150228003340  Create stats table
+```
+
+For large projects with lots of migrations, this is going to be a lot of
+output, so you can trim it down with a simple `grep`:
+
+```bash
+$ rake db:migrate:status | grep '^  down'
+  down    20150228003340  Create stats table
+```