Create LNDHUB.MD

docs/LNDHUB.MD

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+## lndhub
+
+### in the `settings` table
+- set 'LNDHUB_URL' to 'lndhub.yourdomain.com'
+- set 'FUNCTION_LNDHUB' to 'ENABLE'
+
+### create card
+- set the card_name to the 'login:password' for the lndhub account  
+e.g. '11111111111111111111:22222222222222222222'
+
+### limits
+- there are currently no payment rules in this code, only the lndhub account limit  
+i.e. tx_limit_sats & day_limit_sats are not enforced