Mention ProgressTracker near list of template keys

src/lib.rs

@@ -214,7 +214,9 @@
 //! * `eta`: the remaining time (like `elapsed`).
 //! * `duration_precise`: the extrapolated total duration (like `elapsed_precise`).
 //! * `duration`: the extrapolated total duration time (like `elapsed`).
-
+//!
+//! If the list above does not contain the value you need, consider creating a custom
+//! [`ProgressTracker`][crate::style::ProgressTracker] implementation.
 //!
 //! The design of the progress bar can be altered with the integrated
 //! template functionality.  The template can be set by changing a