Add coveralls badge back to README

[jcalcaben]

Description

Adds the Coveralls badge back to the README

Related Issue

Closes PWA-1012

Acceptance

Anyone on the team

Verification Stakeholders

Anyone on the team

Specification

Verification Steps

1. View the README.md file
2. Verify Coveralls badge appears at the top with accurate coverage

Screenshots / Screen Captures (if appropriate)

Checklist

• I have added tests to cover my changes, if necessary.
• I have added translations for new strings, if necessary.
• I have updated the documentation accordingly, if necessary.

[PWAStudioBot]

	Messages
📖	Associated JIRA tickets: PWA-1012.
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).
📖	Access a deployed version of this PR here. Make sure to wait for the "pwa-pull-request-deploy" job to complete.

Generated by 🚫 dangerJS against 35ba0b7

[dpatil-magento]

Confirmed with CI build https://coveralls.io/builds/36650787