DOC: documented the marketplace structure

marketplace/README.md

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+This module contains smart contracts for the data marketplace.
+It was generated with Truffle. Truffle can be used to compile and test
+the smart contracts. It's being tested on a test blockhain using Ganache.
+
+Steps to test:
+* Download an run Ganache
+* Ensure that your Ganache endpoint matches `truffle.js`
+* In this `marketplace` folder:
+    * `truffle compile`
+    * `truffle test`
+    * `truffle migrate --reset`
+    * `truffle console`
+    * From the console: `Marketplace.deployed()`
+* The deployed method displays info about the deployed smart contract
+including its address. Copy/paste the address into 
+`catalyst/marketplace/marketplace.py`.
+* Run the catalyst marketplace unit tests: 
+`tests/marketplace/test_marketplace.py`