Updated index.md from a stub

[alittlepickle]

There is now an actual introduction that gives users a heads-up of the challenges following it. There is also a reminder that help can be found in the Forum, which is linked.

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)
• My pull request targets the master branch of freeCodeCamp.
• None of my changes are plagiarized from another source without proper attribution.
• My article does not contain shortened URLs or affiliate links.

[RandellDawson]

Closed and Reopened this PR to attempt to resolve a specific Travis build failure.

[RandellDawson]

Thank you for opening this pull request.

This is a standard message notifying you that we’ve reviewed your pull request and have decided not to merge it. We would welcome future pull requests from you.

Reason: Since your PR was created, the master branch content has updated making some of your changes unnecessary or need to be reorganized to fit the current version. Please re-sync your fork so you are looking at the latest version of master and then create a new PR if you find any changes warranted.

Thank you and happy coding.