docs: add baseURL for accessing endpoints

[luqmanoop]

Description

This PR updates the README to include baseURL for accessing API endpoints.

Why?
I tried getting the list of phone numbers from https://randomdial.herokuapp.com/phonenumbers and got an error. I thought it'd be nice to have the baseURL clearly stated in the README (I've seen the docs here).

Type of change

Please delete options that are not relevant

• New task (non-breaking change which helps improve the system)
• New feature (non-breaking change which adds functionality)
• New File change/update
• Documentation
• Bug Fix (non-breaking change which fixes an issue)
• This change requires a documentation change

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• End to end
• Unit Test

Checklist:

• My code follows the style guidelines of this project
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing end-2-end tests pass locally with my changes

ISSUE

N/A