feat(README) Point users to the tutorial project [ci skip]

[kbuzzard]

I think the tutorial project is a good place to start, and if other people don't think it is then I think they might want to consider adding more files to the tutorial project. I think mathlib is intimidating for beginners and this is a much better idea. However the link to the tutorial project is not even available on the main page -- you have to click through an installation procedure and find it at the bottom, and even then the first thing is suggests is that you make a new project, which I think is harder than getting the tutorial project up and running. This PR proposes that we point people directly to the tutorial project -- they will probably notice the existence of the tutorial project before they have even installed Lean/mathlib and will hence have it at the back of their mind once they've got things up and running.

TO CONTRIBUTORS:

Make sure you have:

• reviewed and applied the coding style: coding, naming
• reviewed and applied the documentation requirements
• for tactics:
  • added or adapted documentation in tactics.md
  • write an example of use of the new feature in tactics.lean
• make sure definitions and lemmas are put in the right files
• make sure definitions and lemmas are not redundant

If this PR is related to a discussion on Zulip, please include a link in the discussion.

For reviewers: code review check list

[fpvandoorn]

looks good to me

[bryangingechen]

Mergify doesn't merge unless Travis reports back green and [ci skip] in the commit message prevents that.

[fpvandoorn]

Apparently Travis still checks something when I added [ci skip], but it went through eventually.

[bryangingechen]

Ah, you're right, I think my merge was unnecessary in this case. I didn't realize that mergify was waiting for another PR.