readME: Update ReadME for local installation as there is change in .env

[kushthedude]

Describe the bug

Since now the .env.example points to heroku server, Change the readME if the user wants to work on server too.

Current

By default, the .env.example file specifies the API_HOST as localhost:5000 and assumes, that the open-event-server is already up and running. If you intend to work on issues which involve both the frontend, and the backend, please install and set it up first before proceeding to run the frontend. If however, you intend to work on just the frontend, please change the URL for API_HOST to https://open-event-api-dev.herokuapp.com which is a test deployment of the open-event-server.

Change it according to latest

[theaquarium]

Should I just switch them and say that the default is now on Heroku and say that if someone wants to work on both frontend and backend, they should set it up and change the API_HOST to localhost?