Document real-world behaviour

[racitup]

It seems from my use and several other threads (e.g. #73, #69, #57) that this module is far from perfect and there should be some guidance on best practice. For example:

• Which function option(s) could cause you to be blacklisted?
• What can you do to prevent blacklisting?
• How can you tell if you are blacklisted?
• What is the result of the check_mx and verify options if you are blacklisted?
• What is the result of the verify option if the dest. SMTP server doesn't support verify?
• Other cases for false positives (e.g. no tld on a test for publicly routable addresses when using no function options) and false negatives?

This is important for those trying to verify large quantities of external email addresses (in a business, like I am) who need to know the reliability of the results.

It would be great if those most experienced could document this in the README.