doc: release: 2.6: Add notes about rf2xx driver changes

Add 2.6.0 release notes for enhancements on Atmel at86rf2xx driver.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>
diff --git a/doc/releases/release-notes-2.6.rst b/doc/releases/release-notes-2.6.rst
index 6c3ef7a..75bd946 100644
--- a/doc/releases/release-notes-2.6.rst
+++ b/doc/releases/release-notes-2.6.rst
@@ -341,6 +341,13 @@
 
 * IEEE 802.15.4
 
+  * rf2xx:
+
+    * Added support for tx mode direct.
+    * Added support for tx mode CCA.
+    * Added support to enable promiscuous mode.
+    * Added support to enable pan coordinator mode.
+
 * Interrupt Controller
 
 * IPM