Docu on how to ensure foreign() works

tons of people are doing it wrong every day - this should fix it or at
least can be used as reference.

laravel/documentation/database/schema.md

@@ -145,3 +145,9 @@ You may also specify options for the "on delete" and "on update" actions of the
 You may also easily drop a foreign key constraint. The default foreign key names follow the [same convention](#dropping-indexes) as the other indexes created by the Schema builder. Here's an example:
 
 	$table->drop_foreign('posts_user_id_foreign');
+
+> **Note:** The field referenced in the foreign key is very likely an auto increment and therefore automatically an unsigned integer. Please make sure to create the foreign key field with **unsigned()** as both fields have to be the exact same type, the engine on both tables has to be set to **InnoDB**, and the referenced table must be created **before** the table with the foreign key.
+
+	$table->engine = 'InnoDB';
+
+	$table->integer('user_id')->unsigned();
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