wellknown.md: Clarify WELLKNOWN variable

Keep registration information as json.

Use of 'grep -o' doesn't work on SunOS (fixes #164)

In some cases you need your registration id,
for example if you want to request a rate limit exception.
So keep the new-reg output on disk.

Add hook for unchanged certificates.

Example of DNS record

It took me quite a long time to work out the format the DNS record needed to be in. So I have made it crystal clear here.

allow spaces in path to additional configuration file

[doc] Update README.md