Add a basic docstring to the Params module

src/param.rs

@@ -1,4 +1,8 @@
-//! TODO: Document how to use the [`Param`] trait. For now, just look at the gain example.
+//! NIH-plug can handle floating point, integer, boolean, and enum parameters. Parameters are
+//! managed by creating a struct deriving the [`Params`][internals::Params] trait containing fields
+//! for those parmaeter types, and then returning a reference to that object from your
+//! [`Plugin::params()`][crate::prelude::Plugin::params()] method. See the `Params` trait for more
+//! information.
 
 use std::fmt::Display;