[CARBONDATA-2478] Added datamap-developer-guide.md file to Readme.md
[CARBONDATA-2478] Added datamap-developer-guide.md file to Readme.md

[CARBONDATA-2478] Added datamap-developer-guide.md file to Readme.md

This closes #2305

sgururajshetty/carbondata,sgururajshetty/carbondata,sgururajshetty/carbondata