Add 20k devices to readme (#209)

Author: pepijndevos

readme.md

@@ -16,6 +16,8 @@ Currently supported boards are
  * Sipeed Tang Nano 1K: GW1NZ-LV1QN48C6/I5
  * Sipeed Tang Nano 4K: GW1NSR-LV4CQN48PC7/I6
  * Sipeed Tang Nano 9K: GW1NR-LV9QN88PC6/I5 [^1]
+ * Sipeed Tang Nano 20K: GW2AR-LV18QN88C8/I7
+ * Sipeed Tang Primer 20K: GW2A-LV18PG256C8/I7
  * Seeed RUNBER: GW1N-UV4LQ144C6/I5
  * @Disasm honeycomb: GW1NS-UX2CQN48C5/I4