add links for modules and clients

docs/clients/cli.md

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+# CLI
+
+See `gaiacli --help` for more details.
+
+Also see the [testnet
+tutorial](https://github.com/cosmos/cosmos-sdk/tree/develop/cmd/gaia/testnets).

docs/clients/keys.md

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+# Keys
+
+See `gaiacli keys --help`.
+
+Also see the [testnet
+tutorial](https://github.com/cosmos/cosmos-sdk/tree/develop/cmd/gaia/testnets).

docs/clients/node.md

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+# Running a Node
+
+TODO: document `gaiad`
+
+Options for running the `gaiad` binary are effectively the same as for `tendermint`. 
+See `gaiad --help` and the 
+[guide to using Tendermint](https://github.com/tendermint/tendermint/blob/master/docs/using-tendermint.md) 
+for more details.
+
+

docs/clients/rest.md

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+# REST
+
+See `gaiacli advanced rest-server --help` for more.
+
+Also see the 
+[work in progress API specification](https://github.com/cosmos/cosmos-sdk/pull/1314)