Add fee payment ideation (closes cosmos#85)

ideation/fee-payment.md

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+Notes from IBC ecosystem call:
+
+1. Class zero: user = relayer
+    - User relays their own transactions
+    - Zero trust
+    - Could run multiple light clients on a phone, auto-balance funds between fee paying accounts
+1. Class one-half: wallet pays fees
+    - Wallet operator charges extra initial fees, sends all the transactions
+    - Limited trust in the operator
+1. Class one: in-protocol, relayer market
+    - Payment on original chain to relayers
+    - Payment routed through multi-hop IBC transaction, little bits split off for each tx
+    - Zero trust, probably
+1. Class two: out-of-band, specific relayers
+    - ILP payments to relayers which pay fees
+    - Varying amounts of trust
+1. Class three: no fees
+    - DEX accepting incoming liquidity
+    - Zero trust
+
+Formats
+1. Wrap datagram with fee payment
+    1. Fee payment executed, then datagram
+1. Bare datagram, no fees, external account req’d
+    1. Datagram executed as normal
+
+Further ideas from discussion in second ecosystem call:
+
+- Use another chain to coordinate the relayers
+- Slash if a leader fails to relay (after some time period)
+- Prior fee payment sent to relayer chain
+- Relayers maintain balance everywhere, payment channels
+- Latency mechanism for relayer accountability, slashing on failure to meet latency obligations
+- Chains negotiate on relayer payments with the relayer zone
+- “IBC relayer fee negotiation protocol”