schema Image link corrected in Readme.md

Fixed Schema image link in Architecture section.

## Earlier - 

![schema_old](https://cloud.githubusercontent.com/assets/4817493/5196994/403e6c9e-7561-11e4-8097-990627952cbe.png)

## Modified - 

![schema_new](https://cloud.githubusercontent.com/assets/4817493/5197002/507472ca-7561-11e4-90ca-787af88662f1.png)