Add 1-2-3 instructions for new contributors to front page of web

[stefanv]

No description provided.

[soupault]

@stefanv could you elaborate on this, please?

[07harshita]

Hi!
I would love to work on this issue.

[yashasvimisra2798]

@stefanv can you please elaborate on what exactly needs to be added, I want to work on this issue

[yashasvimisra2798]

Is this issue relevant right now?

[stefanv]

Hi @yashasvimisra2798; thank you for your willingness to help out!

The idea here was to have a very short version of the contributor guide that showed the minimum steps required to contribute a change to skimage. I.e., clone the repo, make a branch, commit to that branch, make a pull request.

Perhaps this should be a summary at the top of the contributor guide with simply a link from the front page? Either way, the intent is to make contribution look straightforward and not scary!