Add docs on SSL certificates

[florimondmanca]

Fixes #507

[yeraydiazdiaz]

Really cool we're adding this, it confused the heck out of me every time 😄

Left some questions and nitpicks on phrasing.

[florimondmanca]

@yeraydiazdiaz I pushed some commits addressing your comments, feel free to give this another look. 😄

[lovelydinosaur]

I really think it's just the client certificates that come under the "you probably don't need this" disclaimer category.

[florimondmanca]

@tomchristie Yeah, makes sense. I figured we could just drop the warning — after all, the whole page is labelled "Advanced Usage", right? 😄

[yeraydiazdiaz]

LGTM thanks @florimondmanca

[florimondmanca]

Mental note that it seems @sethmlarson might have trustme-cli brought into trustme directly (see sethmlarson/trustme-cli#1), so we’ll want to update the link here when that happens. :)