add link to contributors, remove duplicate "to"s
Merge pull request #193 from waldyrious/patch-1

add link to contributors, remove duplicate "to"s
revise the description of A records on domains: the A record must be present for good deliverability so that the envelope domain resolves, but it doesn't have to resolve to this machine
correct typo in comment
python3-dev is a dependency for many pip packages, including pyyaml, fixes #196