tween docs: flesh out description, define all arguments (#720)

Author: thetarnav

packages/tween/README.md

@@ -9,7 +9,8 @@
 [![size](https://img.shields.io/npm/v/@solid-primitives/tween?style=for-the-badge)](https://www.npmjs.com/package/@solid-primitives/tween)
 [![stage](https://img.shields.io/endpoint?style=for-the-badge&url=https%3A%2F%2Fraw.githubusercontent.com%2Fsolidjs-community%2Fsolid-primitives%2Fmain%2Fassets%2Fbadges%2Fstage-3.json)](https://github.com/solidjs-community/solid-primitives#contribution-process)
 
-Creates an efficient tweening function.
+Creates an efficient tweening derived signal that smoothly transitions
+a given signal from its previous value to its next value whenever it changes.
 
 ## Installation
 
@@ -22,9 +23,46 @@ yarn add @solid-primitives/tween
 ## How to use it
 
 ```ts
-const tweenedValue = createTween(myNumber, { duration: 500 });
+import { createSignal } from "solid-js";
+import { createTween } from "@solid-primitives/tween";
+
+const [value, setValue] = createSignal(0);
+const tweenedValue = createTween(value, { duration: 500 });
+
+setMyNumber(100);
+// tweenedValue will now smoothly transition from 0 to 100 over 500 ms
+```
+
+## Definition
+
+```ts
+function createTween(
+  target: Accessor<number>,
+  options: {
+    duration?: number = 100; // ms
+    easing?: (t: number) => number = (t) => t;
+  }
+): Accessor<number>;
 ```
 
+`target` can be any reactive value (signal, memo, or function that calls such).
+For example, to use a component prop, specify `() => props.value`.
+
+You can provide two options:
+
+* `duration` is the number of milliseconds to perform the transition
+  from the previous value to the next value. Defaults to 100.
+* `easing` is a function that maps a number between 0 and 1 to a number
+  between 0 and 1, to speed up or slow down different parts of the transition.
+  The default easing function `(t) => t` is linear (no easing).
+  A common choice is `(t) => 0.5 - Math.cos(Math.PI * t) / 2`.
+
+Internally, `createTween` uses
+[`requestAnimationFrame`](https://developer.mozilla.org/en-US/docs/Web/API/window/requestAnimationFrame)
+to smoothly update the tweened value at the display refresh rate.
+After the tweened value reaches the underlying signal value,
+it will stop updating via `requestAnimationFrame` for efficiency.
+
 ## Changelog
 
 See [CHANGELOG.md](./CHANGELOG.md)