rdm/radio_hal: add radio HAL design document

Author: jia200x

doc/memos/README.md

@@ -4,6 +4,7 @@ The index below lists the RDMs that were published so far.
 
 - [RDM0 : RIOT Developer Memo Format, Publishing and Maintenance Process](./rdm0000.md)
 - [RDM1 : RIOT Design Goals](./rdm0001.md)
+- [RDM2 : The 802.15.4 Radio HAL](./rdm0002.md)
 
 # RDM development process