make it clear how to enable the take method

Author: Emilgardis

src/lib.rs

@@ -170,7 +170,7 @@
 //! To use a peripheral first you must get an *instance* of the peripheral. All the device
 //! peripherals are modeled as singletons (there can only ever be, at most, one instance of any
 //! one of them) and the only way to get an instance of them is through the `Peripherals::take`
-//! method.
+//! method, enabled via the `critical-section` feature on the generated crate.
 //!
 //! ```ignore
 //! let mut peripherals = stm32f30x::Peripherals::take().unwrap();