Beta 2 migration docs

docs/4.0/migration.md

@@ -6,9 +6,22 @@ group: migration
 toc: true
 ---
 
-{% callout info %}
-**Heads up!** This will be in flux as work on the v4 alphas progresses. Until then consider it incomplete, and we'd love pull requests to help keep it up to date.
-{% endcallout %}
+## Beta 2 changes
+While in beta, we aim to have no breaking changes. However, things don't always go as planned. Below are the breaking changes to bear in mind when moving from Beta 1 to Beta 2.
+
+### Breaking
+
+- Removed `$badge-color` variable and its usage on `.badge`. We use a color contrast function to pick a `color` based on the `background-color`, so the variable is unnecessary.
+- Renamed `grayscale()` function to `gray()` to avoid breaking conflict with the CSS native `grayscale` filter.
+- Renamed `.table-inverse`, `.thead-inverse`, and `.thead-default` to `.*-dark` and `.*-light`, matching our color schemes used elsewhere.
+- Responsive tables now generate classes for each grid breakpoint. This breaks from Beta 1 in that the `.table-responsive` you've been using is more like `.table-responsive-md`. You may now use `.table-responsive` or `.table-responsive-{sm,md,lg,xl}` as needed.
+- Dropped Bower support as the package manager has been deprecated for alternatives (e.g., Yarn or npm). [See bower/bower#2298](https://github.com/bower/bower/issues/2298) for details.
+- Bootstrap now requires jQuery 3.0.0 or higher.
+
+### Highlights
+
+- Introduced new `pointer-events` usage on modals. The outer `.modal-dialog` passes through events with `pointer-events: none` for custom click handling (making it possible to just listen on the `.modal-backdrop` for any clicks), and then counteracts it for the actual `.modal-content` with `pointer-events: auto`.
+
 
 ## Summary