doc: Add reference doc about feature needed to use c sub-module

jhg · jhg · commit b931e37fff11 · 2021-01-29T12:55:59.000+01:00
diff --git a/src/c.rs b/src/c.rs
@@ -1,4 +1,6 @@
 //! # Specific C types like C-like string pointers.
+//!
+//! You need to use the `c-types` feature to use this module.
 
 #![cfg(all(feature = "std", feature = "c-types"))]
 
