Update README.md

[ivancheban]

Changed the headings to sentence-case for consistency and style reasons.

{PR title}

• You've read the Contributor Guide and Code of Conduct.
• You've included unit or integration tests for your change, where applicable.
• You've included inline docs for your change, where applicable.
• There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.

Summary of the changes (Less than 80 chars)

Description

{Detail}

Fixes #{bug number} (in this specific format)

[ghost]

Thanks for your PR, @ivancheban. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[mkArtakMSFT]

Thanks for your PR, @ivancheban.

[ghost]

Hey @dotnet/aspnet-build, looks like this PR is something you want to take a look at.