Add description to readme of openmrs-module-metadatamapping

Add description to readme of openmrs-module-metadatamapping: https://github.com/openmrs/openmrs-module-metadatamapping .

Resource:  https://wiki.openmrs.org/display/docs/Metadata+Mapping+Module

Submission Steps:

1) git clone https://github.com/openmrs/openmrs-module-metadatamapping

2) update readme

3) create pull request and submit for code review