Update 'raspi.md'

raspi.md

@@ -1,6 +1,9 @@
 Raspberry Pi using letsencrypt:
 
 `gpg --recv-key 7638D0442B90D010`
+`gpg --recv-keys 8B48AD6246925553`
+`gpg -a --export 7638D0442B90D010 | sudo apt-key add -`
+`gpg -a --export 8B48AD6246925553 | sudo apt-key add -`
 
 `git clone https://github.com/letsencrypt/letsencrypt`