#docs: snippets for system; dmenu-session

created document section in Miscellaneous section "Snippets for the system" with example. It is test push to test how github works :)

And please write what is wrong here and what I should do

P.S. it is moved to branch what I could do other stuff. Old pull request https://github.com/qtile/qtile/pull/353 . It could be closed.