Add a mention to the foreign_key argument of has_many and belongs_to.
//!
//! [Identifiable]: trait.Identifiable.html
//!
+//! If the name of your foreign key doesn't follow the convention `tablename_id`, you can specify a
+//! custom one to `#[has_many]` and `#[belongs_to]` by adding a `foreign_key` argument to the
+//! attribute like so `#[has_many, foreign_key="mykey"]`.
+//!
//! `#[has_many]` actually has no behavior on its own. It only enables joining between the two
//! tables. If you are only writing `children.inner_join(parents)` or
//! `Child::belonging_to(&parents)`, you only need to define the `#[belongs_to]` side.
