report disk usage where the data actually lives

[mrq1911]

fixes #5.

DiskUsage hardcoded the platform root, so nodes with data on a separate partition reported the wrong figures.

• new DISK_PATH env var, default / (\ on windows) — payload shape is unchanged, so no swarmpit-side change needed
• log instead of silently returning zeroes when the path can't be read, since a typo'd DISK_PATH is now a way to get 0% disk

verified against a real second mount — DISK_PATH=/mnt/data on a 64m tmpfs reports total=67108864 where root reports total=1996001169408.

build, vet and tests pass on golang:1.12. tests cover the default, the override, and a bad path returning zeroes rather than panicking.

[mrq1911]

pushed a second commit documenting DOCKER_HOST / DOCKER_API_VERSION / tls in the same readme block — it answers #10 (already supported, just undocumented) and the config trap behind #13 and #14. put it here rather than in its own branch since it edits the same lines as the DISK_PATH entry.