Update README.md

[manavluhar]

Description (*)

• Added " • " before Magento System Requirements Link to give generalize look to : Magento System Requirements and Install Magento, both the sections.

• Removed full stops(.) from Magento System Requirements and Install Magento Links.

• As Contribution Guide is common for all versions at DevDocs, changed the link for the same from https://devdocs.magento.com/guides/v2.3/contributor-guide/contributing.html to https://devdocs.magento.com/contributor-guide/contributing.html

• Changed https://devdocs.magento.com/guides/v2.3/contributor-guide/contributing.html#report this link with https://devdocs.magento.com/contributor-guide/contributing.html#report as before one was not redirecting to Report an issue section in DevDocs

• Added this line : To open an issue, click here, in New line.

• Changed https://devdocs.magento.com/guides/v2.3/contributor-guide/contributing.html#labels this link with https://devdocs.magento.com/contributor-guide/contributing.html#labels as before one was not redirecting to Labels applied by the Community Engineering team section in DevDocs

• Changed https://github.com/magento/security-package/blob/2.3-develop/LICENSE.txt this link with https://github.com/magento/security-package/blob/develop/LICENSE.txt as before one was redirecting to 404 page

• Added updated link for Slack channels, Which will redirect the user to the Slack Application instead of opening the Slack channel in the browser. If the user is not having a Slack Application, then the Slack channel will open in the browser. The purpose is to gain more traffic on the Slack Channel.

Fixed Issues (if relevant)

1. magento/magento2#<issue_number>: Issue title

Manual testing scenarios (*)

1. ...
2. ...

Questions or comments

Contribution checklist (*)

• Author has signed the Adobe CLA
• 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 are green)