Reinforce Contributing Guidelines for each newly created issue

[julien-deramond]

Description

This PR adds a GitHub action triggered by any issue created in the twbs/bootstrap repository. The action will append a horizontal rule and a message reinforcing our contributing guidelines to the issue body by saying that we don't assign issues to contributors.

The rendering of this GitHub Action can be observed in the following issues:

• Blank issue Open-reSource/labs-append-markdown-to-issues#17 (blank issue)
• Issue created via issue template Open-reSource/labs-append-markdown-to-issues#18 (issue created via a template)

The content of the Yaml file is almost the same as the one suggested in this PR: https://github.com/Open-reSource/labs-append-markdown-to-issues/blob/main/.github/workflows/append-markdown-to-issues.yml.

Everything is based on a GitHub Action now available on the Marketplace: https://github.com/julien-deramond/update-issue-body/tree/v1.0.0.

It hasn't yet been used by any other repository, so I'd understand if you'd like to wait for first use cases somewhere else before integrating it in the Bootstrap repository.

Motivation & Context

This message will hopefully reduce the amount of noise in our issue tracker and help contributors to understand our contributing guidelines.

Type of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• (N/A) My change introduces changes to the documentation
• (N/A) I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• All new and existing tests passed