Add disclaimer about subspaces being experimental in add existing dialog

res/css/views/dialogs/_AddExistingToSpaceDialog.scss

@@ -79,6 +79,32 @@ limitations under the License.
             border-radius: 8px;
         }
     }
+
+    .mx_AddExistingToSpace_section_experimental {
+        position: relative;
+        border-radius: 8px;
+        margin: 12px 0;
+        padding: 8px 8px 8px 42px;
+        background-color: $header-panel-bg-color;
+
+        font-size: $font-12px;
+        line-height: $font-15px;
+        color: $secondary-fg-color;
+
+        &::before {
+            content: '';
+            position: absolute;
+            left: 10px;
+            top: calc(50% - 8px); // vertical centering
+            height: 16px;
+            width: 16px;
+            background-color: $secondary-fg-color;
+            mask-repeat: no-repeat;
+            mask-size: contain;
+            mask-image: url('$(res)/img/element-icons/room/room-summary.svg');
+            mask-position: center;
+        }
+    }
 }
 
 .mx_AddExistingToSpaceDialog {

src/components/views/dialogs/AddExistingToSpaceDialog.tsx

@@ -120,6 +120,10 @@ export const AddExistingToSpace: React.FC<IAddExistingToSpaceProps> = ({ space,
             { spaces.length > 0 ? (
                 <div className="mx_AddExistingToSpace_section mx_AddExistingToSpace_section_spaces">
                     <h3>{ _t("Spaces") }</h3>
+                    <div className="mx_AddExistingToSpace_section_experimental">
+                        <div>{ _t("Feeling experimental?") }</div>
+                        <div>{ _t("You can add existing spaces to a space.") }</div>
+                    </div>
                     { spaces.map(space => {
                         return <Entry
                             key={space.roomId}

src/i18n/strings/en_EN.json

@@ -2031,6 +2031,8 @@
     "Matrix rooms": "Matrix rooms",
     "Filter your rooms and spaces": "Filter your rooms and spaces",
     "Spaces": "Spaces",
+    "Feeling experimental?": "Feeling experimental?",
+    "You can add existing spaces to a space.": "You can add existing spaces to a space.",
     "Direct Messages": "Direct Messages",
     "Space selection": "Space selection",
     "Add existing rooms": "Add existing rooms",