readme: GnuPG v1.x errors on hkps scheme

[xaur]

gpg --keyserver hkps://pgp.mit.edu --recv-key 0x14B9CD80

gives

HTTP fetch error 1: unsupported protocol

It works if I replace hkps with hkp or remove the scheme entirely or if I use GnuPG v2. Web search tells it will also work after installing gnupg-curl package, and it also suggests this is a common issue.

So there are 2 issues with hkps: it doesn't work on older default setups and (if I got it right) it relies on CA infrastructure which I'm not sure is a good or bad thing.