README: separate CAs hardening documentation

[nodiscc]

This is based on #239 and adds a single commit/change:

• split CAs hardening guide to separate file (greatly improves readability of README),
• Increase CA problem visibility and link to it from 'Further hardening' section
• deduplicate usage information, link to appropriate sections instead of repeating ourselves

---

I believe this is a much needed change to get users/contributors to

• actually read the whole readme
• actually install user.js without getting rebutted by the apparent complexity of the procedure
• seriously consider trimming down CA lists as a further hardening step

Preview at:

https://github.com/nodiscc/user.js/blob/readme-separate-cas-doc/README.md#further-hardening
https://github.com/nodiscc/user.js/blob/readme-separate-cas-doc/CAs.md

[pyllyukko]

I agree that the CAs section might be a bit overkill for the main README. Also looking at the issues/PRs, there doesn't seem to be much attention against it.

It's still an important part of hardening the browser, but it might be better to separate it.

So apparently we should merge either this or #239?

[nodiscc]

So apparently we should merge either this or #239?

We should wait for #239 to be merged, then merge this (or git cherry-pick 3974a01 but I don't think it can be applied on top of current master that easily, git will probably complain and request manual editing)

[nodiscc]

Rebased.