modifiers: add short README

packages/modifiers/README.md

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+# modifiers
+
+Use modifiers to change the coordinates of drag, resize and gesture events.
+
+The `options` object passed to the action methods can have a `modifiers` array
+which will be applied to events of that action type.
+
+```js
+// create a restrict modifier to prevent dragging an element out of its parent
+const restrictToParent = interact.modifiers.restrict({
+  restriction: 'parent',
+  elementRect: { left: 0, right: 0, top: 1, bottom: 1 },
+})
+
+// create a snap modifier which changes the event coordinates to the closest
+// corner of a grid
+const snap100x100 = interact.modifiers.snap({
+  targets: [interact.snappers.grid({ x: 100, y: 100 })],
+}),
+
+// apply the restrict and then the snap modifiers to drag events
+interact(target).draggable({
+  modifiers: [restrictToParent, snap100x100],
+})
+```