Add docs in EnvVar section for `mode`
Add docs in EnvVar section for `mode`

Fixes: #117 by adding docs for the mode env variable

Signed-off-by: Alistair Hey <e9adbcf81c8a36ccc628e09bc92d0e25e1c8e501@heyal.co.uk>

openfaas-incubator/of-watchdog,openfaas-incubator/of-watchdog