Update main index.md

[fizisist]

The information in the index.md file appears on the main page of the website, and so should include a broad description of the Coding Club as well as a straightforward links to the Starter Kit, Tutorials, and Projects pages.

Links to pages can be included as relative paths (see example in the index.md page, as well as the "back" link on the StarterKit page).

[sidneymau]

This has been completed and is in a working state.