[docs] Update Available Flags

[elithrar]

The current README shows the flags in GNU getopt_long style as a holdover from the original C version of Keyless (e.g. https://github.com/cloudflare/gokeyless#key-management)

We should update these and document new flags (i.e. -manual-activation) offered by gokeyless.

[lziest]

fixed.