Update README.md

Author: analyticsariel

rental_estimates/README.md

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 # Rental Estimates
 This directory provides a "how to" guide on retrieving rental estimates from different APIs.
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+## Sources
+1. [RealtyMole | Property Rent Estimate Endpoint](https://rapidapi.com/realtymole/api/realty-mole-property-api/)
+2. [Rentometer | Rent Summary Endpoint](https://www.rentometer.com/developers/api_docs#rent-summary-quickview)
+3. [US Census | ACS dataset](https://www.census.gov/data/developers/data-sets/acs-5year.html)
+4. [Zillow | Quandl API](https://data.nasdaq.com/databases/ZILLOW/data)