Document portable options' mode in both the server and viewer?

[lwcorp]

How to enable portable mode in the first place?

Originally posted by @lwcorp in #353

winvnc
-Create an empty file named ultravnc.portable in the same folder as winvnc.exe, then it read the ini from the same folder ( old behavior)

Viewer has no portable mode
The viewer try to read the options.vnc in the current folder ( just like before)
Only when that doesn't exist the new %appdata% path is used.
In options.vnc you can set the log and snapshot path

Originally posted by @RudiDeVos in #353

Thanks! Could you document it? Maybe in https://uvnc.com/docs/documentation.html

Originally posted by @lwcorp in #353