Rework README to point users to rustls-platform-verifier

[djc]

Alternative to #140, which IMO is too cautious.

cc @complexspaces

(The screenshot is mostly for showing the way it renders -- review the diff for current text.)

[cpu]

I like the fancy markdown rendering of the important note 👍

[complexspaces]

I'm fine with this as an alternative to my suggested changes! I was hesitant to do it myself since I thought we were not quite ready to make the recommendation cutover but I have no issues with the idea.

[djc]

I thought we were not quite ready to make the recommendation cutover

Any particular reason? IMO this note is unlikely to affect existing users much and we already have some adoption (for example in Quinn, and mainline rustup).

[complexspaces]

I had this comment in mind, specifically this part:

Given my experience so far I think it is too early to do anything like this. On Linux and BSDs, there is basically no benefit to using rustls-platform-verifier, and for now the adoption of rustls-platform-verifier doesn't appear to be taking off (for the past 90 days, the platform verifier got 6% of the downloads compared to native-certs).

The new wording here is basically a soft-deprecation of the crate.

[djc]

Right. I think the deprecation should be more of a slightly lagging indicator whereas advice from this README seeks to be leading. We've also made some progress since then with the additivity issue.

[complexspaces]

That sounds fine to me 👍 .

[djc]

I've fixed the wording as pointed out by @ctz and split this in more commits to make it clearer why things changed. Please have another look!

[cpu]

LGTM, thanks djc