Add EngCom Slack info to README and formatting

[lorikrell]

Description (*)

No code changes
Add Community Engineering SLACK info to README
Minor formatting updates to page

Fixed Issues (if relevant)

No issue associated
EngCom request

Manual testing scenarios (*)

No testing need, no code change

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @lorikrell. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me 2.3-develop instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[magento-engcom-team]

Hi @okorshenko, thank you for the review.
ENGCOM-3662 has been created to process this Pull Request

[magento-engcom-team]

Hi @lorikrell. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.