README.md: suggest using ~all instead of -all

If you use an SPF record like v=spf1 ip4:xxx.xxx.xxx.xxx -all mail-tester will deduct 2 points, saying

You have ?all in your SPF, instead of ~all. This means we can't verify if your SPF is good, although it might probably be!

This may be a bug with mail-tester, but according to this it is generally harmless to use ~all: https://wordtothewise.com/2014/06/authenticating-spf/

Note that I don't really know much about what is the best practice here. @r-raymond is there a particular reason you went with -all?