Clarify the function of export_base_url in the documentation

[eyedeekay]

It would help people starting to use the skf if the documentation mentioned that the export_base_url can accept a value which includes the domain, like:

export_base_url=https://user.github.io/

or with the domain and the path to the skf site's folder

export_base_url=https://user.github.io/my_skf_blog

or with just the path to the skf site's folder(Including leading slash)

export_base_url=/my_skf_blog/

or, if left with no value at all it will also work. I noticed this because I was attempting to test locally and it redirected me to a nonexistent page at my github blog. Including a few more examples like the ones above would help users who might want to present their site on different domains(Similarly, static sites are especially easy to deploy as Tor Hidden Services or i2p eepSites, which I personally think are the most fun way to blog.) or test things on a LAN before putting them online.

[moebiuseye]

Will add an explanation with a reference to this issue. Thank you very much for contributing! :)