README: Mention the --demo-mode parameter needed to run a sandbox.

Author: Sventimir

README-sandbox.md

@@ -148,6 +148,7 @@ $ mina daemon \
     --proof-level none \
     --block-producer-pubkey "$(cat keys/block-producer.key.pub)" \
     --run-snark-worker "$(cat keys/snark-producer.key.pub)" \
+    --demo-mode \
     --seed
 ```
 
@@ -172,6 +173,11 @@ over again, it's important to remove it before each restart. Otherwise
 the daemon will try to pick up the old blockchain and will get stuck
 on it.
 
+`--demo-mode` option tells the daemon to assume it's already synced
+with the network (because in this case *it is* the entire network).
+Without this option the daemon will exit after approximately 30 minutes
+of being unable to connect to other nodes.
+
 If the block producer's key wasn't copied over to the wallet
 previously, the following error will appear: