Add sigv4/README.md (#336)
Add sigv4/README.md (#336)

I couldn't figure out why sigv4 is a separate Go module, and I imagine
other people might have the same question.

Signed-off-by: Marcelo E. Magallon <56cce93673b8e0eba950695f805812ddc478ba9f@grafana.com>
prometheus/common,m3db/prometheus_common