CONTRIBUTING update for the github squash button.

[gianm]

Some changes to the contributing guidelines to make pull requests
easier to review. The existing guidelines were written back when github
didn't have a squash-and-rebase feature, so people had to squash
their own commits. Now it's easier if they don't.

[fjy]

👍

[leventov]

Another problem is that CONTRIBUTING.md suggests to do rebases, instead of merging master into feature branch. Rebases are very inconvenient for reviewing, because reviewer cannot separately look at changes that he hasn't yet reviewed, and also older comments to the code are lost.

Before Github used to display full diffs of merge commits so it was also inconvenient for reviewing. But now Github always displays diff between the feature branch and the current master, so this problem is gone.

[gianm]

I think the changes address that concern, it now says:

Avoid rebasing and force pushes after submitting a pull request, since these make it difficult for reviewers to see what you've changed in response to their reviews.