I ( @boukendesho ) have volunteered to maintain the snap package so
this adds it back into the installation instructions.
It will set a `snap` tag visible in `rclone version` so we know where
it came from for support queries.
This was necessary because go1.14 seems to have a modules related bug
which means it tries to build modules even though the uses of them are
all disabled with build constraints. This seems to be fixed in go1.15.