docs(readme): API docs for PK.index()

README.md

@@ -2778,15 +2778,31 @@ Generates a string value representative of this key.
 value(): string
 ```
 
+**Description**
+
 This is used for the `equals` method, which is strongly preferred for testing the equality of two keys.
 
-**Parameters**
+**Returns**
 
-None
+`string`: A hash value representative of the key.
+
+##### `index()`
+
+Returns the placement of this key in the list of all keys.
+
+```ts
+index(): number
+```
+
+**Description**
+
+For example, an index of 2 means this was the second key generated.
+
+This includes reserved keys; if a set of reserved keys was used to generate this key's `PKProvider`, the entry with the lowest value will have an index of 0.
 
 **Returns**
 
-`string`: A hash value representative of the key.
+`number`: The index of this key.
 
 ### `RegalError`