Add readme in back and front end repo

[barshathakuri]

For Backend Repo in readme, there is a description of CAP Aggretation instead of it. Please add 

"This repository houses the CAP Aggregator and Alert Manager for IFRC's Alert Hub. These services aggregate and distribute public alerts using the Common Alerting Protocol (CAP) Version 1.2 standard."

You can add more detail from a backend perspective. 

Note: That isn’t a requirement but it might be good practice to just add a oneliner or two for external people