cactus: add fastboot instructions

Change-Id: Icd459ebecc01f71eba62601cff84fb9b3023a801
diff --git a/_xiaomi/xiaomiredmi6a.markdown b/_xiaomi/xiaomiredmi6a.markdown
index b88d4d1..10d6187 100644
--- a/_xiaomi/xiaomiredmi6a.markdown
+++ b/_xiaomi/xiaomiredmi6a.markdown
@@ -20,3 +20,5 @@
 {% include twrpinstall.html %}
 
 {% include mtkinstall.html %}
+
+{% include fastbootinstall.html %}