nx609j: add twrpapp section back

- Update "downloadfolder" contents for both versions

This reverts commit 448059d45feda5b103201ac5628a7b84df67af56.

Change-Id: Ib7a0f1459c59101df3795227787626168f05cc8b
diff --git a/_nubia/nubianx609j.markdown b/_nubia/nubianx609j.markdown
index 8dbcd60..e5e21bf 100644
--- a/_nubia/nubianx609j.markdown
+++ b/_nubia/nubianx609j.markdown
@@ -2,7 +2,7 @@
 layout: device
 title:  "Nubia ZTE 红魔电竞游戏手机 (Red Magic)"
 codename: nx609j
-downloadfolder: nx609j
+downloadfolder: "Oreo firmware (legacy system setup)|nx609j|Pie firmware (system-as-root setup)|nx609j_SAR"
 supportstatus: Current
 maintainer: youyim, Captain_Throwback
 oem: Nubia
@@ -15,6 +15,8 @@
 
 {% include supportstatus.html %}
 
+{% include appinstall.html %}
+
 <div class='page-heading' id='download'>Download Links:</div>
 <hr />
 <p class="text">For Oreo firmware - legacy system setup:</p>