docs: add entry for the declarative columns change (#13)

Author: damyanpetev

CHANGELOG.md

@@ -11,6 +11,41 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- **BREAKING:** Column configuration is now declarative using `<igc-grid-lite-column>` elements instead of the `columns` property.
+  The `columns` property is now read-only and returns the current column configuration.
+
+  Before:
+
+  ```html
+  <igc-grid-lite .data=${data} .columns=${columns}></igc-grid-lite>
+  ```
+
+  ```ts
+  const columns: ColumnConfiguration<User>[] = [
+    { key: 'id', headerText: 'User ID', type: 'number', filter: true, sort: true },
+    { key: 'name', filter: true, sort: true },
+  ];
+  ```
+
+  After:
+
+  ```html
+  <igc-grid-lite .data=${data}>
+    <igc-grid-lite-column
+      key="id"
+      header-text="User ID"
+      type="number"
+      .filter=${true}
+      .sort=${true}
+    ></igc-grid-lite-column>
+    <igc-grid-lite-column
+      key="name"
+      .filter=${true}
+      .sort=${true}
+    ></igc-grid-lite-column>
+  </igc-grid-lite>
+  ```
+
 - **BREAKING:** Renamed `GridSortConfiguration` type to `GridLiteSortingOptions`.
 - **BREAKING:** Renamed `IgcGridLite.sortConfiguration` property to `sortingOptions`.
 - **BREAKING:** Renamed `IgcGridLite.sortExpressions` property to `sortingExpressions`.