[#311] add more info for contributing to Indigo's Website

Description

Problem: Indigo's website does not explain how to open issues or merge requests for it. Additionally, its directory's README does not specify well how to build it or check the result of changes.

Solution: add a paragraph for contributing to the website's main page and more building info in its source code README.

Related issue(s)

Resolves #311 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.