document new prelayout hook

markdown/dev/reference/hooks/prelayout/en.md

@@ -0,0 +1,26 @@
+---
+title: preLayout
+---
+
+The `preLayout` lifecycle hook runs just before the pattern layout is
+calculated.
+
+This is a good place to do any business that might change the layout but relies on all the parts having been drafted
+
+## Signature
+
+```js
+null hook(Pattern pattern)
+```
+
+## Example
+
+```js
+pattern.on('preLayout', pattern => {
+  // Mutate the pattern object here
+}
+```
+
+## Notes
+
+The `postLayout` hook is rarely used, but it's there if you need it.