add notes about picker and filter

Signed-off-by: David An <a-daan@expediagroup.com>

.changeset/funny-trains-sniff.md

@@ -3,4 +3,23 @@
 '@backstage/plugin-catalog': minor
 ---
 
-Added namespace filter and column in default catalog table
+Added an entity namespace filter and column on the default catalog page.
+
+If you have a custom version of the catalog page, you can add this filter in your CatalogPage code:
+
+```ts
+<CatalogFilterLayout>
+  <CatalogFilterLayout.Filters>
+    <EntityTypePicker />
+    <UserListPicker initialFilter={initiallySelectedFilter} />
+    <EntityTagPicker />
+    /* if you want namespace picker */
+    <EntityNamespacePicker />
+  </CatalogFilterLayout.Filters>
+  <CatalogFilterLayout.Content>
+    <CatalogTable columns={columns} actions={actions} />
+  </CatalogFilterLayout.Content>
+</CatalogFilterLayout>
+```
+
+The namespace column can be added using `createNamespaceColumn();`. This is only needed if you customized the columns for CatalogTable.