Citing Obsolete SPF RFC 4408 vice Current RFC 7208

[chrislandis]

I noticed in the documentation that it cites RFC 4408, which RFC 7208 has superseded.

trustymail/README.md

Lines 141 to 142 in 585c9a8

	- `Valid SPF` - Whether the SPF record found is syntactically correct,
	per RFC 4408.

I cannot find anywhere else in the project that cites RFC 4408, but RFC 7208 is cited in #7 (comment) and #21 (comment). Comparing the trustymail code and these two RFCs, it seems that trustymail does not conflict with RFC 7208 and should cite it instead of RFC 4408.

If you would like, I can make this one-line change in the README and submit a pull request.

[jsf9k]

Thank you for the issue, @chrislandis!

Please feel free to go ahead and make the change to the README. While you're at it, we may as well make the RFC 7208 text link to the RFC as well.