Add a note about key expiry setup

Signed-off-by: Elan Ruusamäe <[email protected]>

Author: glensc

README.md

@@ -49,6 +49,7 @@ gpg --edit-key "<YOUR MASTER KEY ID>"
 A signing key type RSA with 2048 bits is recommended for greatest compatibility.
 
 Type `addkey` and select a type that is for signing, you might be asked about bit size depending on your choice.
+When deciding over key expire, avoid setting to never expire, as recommendation of key bits will change over time.
 Type `save` to persist the new subkey to your master key. Make a note of the Key ID  as you will need it in the next step.
 
 Next export the new sub key: