add path to fish code example

Example 1: fish shell add to path

set -Ua fish_user_paths /path/to/add

# fish_user_paths is equivilent to PATH
# -U make variable univeral
# -a append variable

Example 2: fish add to path

set -gx PATH /opt/qt/Tools/QtCreator/bin /opt/qt/5.0.0/gcc_64/bin $PATH