Note that composer.json file should be in project root

What does this MR do?

Clarifies that the composer.json file should be in the project root of the repository for successful publication of a Composer package.

I was unable to publish a test composer package from a subdirectory of a project. It was extremely confusing that I kept receiving The file composer.json was not found error. It turned out that we are checking for composer.json file in the root directory of the repository. It is not possible to publish from a subdirectory right now.

Related issues

• Add composer test to gitlab-smokes-test project

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