Added Foreign Key section

schema.md

@@ -6,6 +6,7 @@
 - [Dropping Columns](#dropping-columns)
 - [Adding Indexes](#adding-indexes)
 - [Dropping Indexes](#dropping-indexes)
+- [Foreign Keys](#foreign-keys)
 
 <a name="introduction"></a>
 ## Introduction
@@ -108,3 +109,38 @@ Command  | Description
 `$table->dropPrimary('users_id_primary');`  |  Dropping a primary key from the "users" table
 `$table->dropUnique('users_email_unique');`  |  Dropping a unique index from the "users" table
 `$table->dropIndex('geo_state_index');`  |  Dropping a basic index from the "geo" table
+
+<a name="foreign-keys"></a>
+## Adding Columns
+
+When building your database you're very likely have columns in one table referencing primary keys of another table, for instance when you're using [Eloquent's Relationships](eloquent#relationships) features. If you let your database engine know this you can leverage it's capacity for keeping this data consistent and working smothly.
+
+> **Note:** Both columns muse be of the same type. Primary keys are unsigned integers so referencing columns need to be `->unsigned()` aswell.
+
+> **Note:** The referenced table must created before the referencing table.
+
+**Add A Foreign Key Constraint**
+
+	$table->foreign("user_id")->references('id')->on('users');
+
+You can also specify what you want the database engine to do when the referenced column is updated or deleted
+
+	$table->foreign("user_id")->references('id')->on('users')->onDelete('restrict');
+
+	$table->foreign("user_id")->references('id')->on('users')->onUpdate('cascade')->onDelete('restrict');
+
+Option  |  Description
+------------- | -------------
+`restrict`  |  Prevents deletion or update of the referenced row
+`cascade`  |  The action replicates - the column is updated to the new value, or the row is deleted along with the referenced row
+`set null`  |  The value is set to null when the foreign key is updated or deleted
+`no action`  |  Error is raised, but the action is performed
+`set default`  | The value is set to the default for the column when the foreign key is updated or deleted
+
+> **Note:** The default option is `no action`
+
+> **Note:** `set default` is not supported by MySQL InnoDB!
+
+**Dropping Foreign Key Constraints**
+
+	$table->dropForeign("user_id");