Add package TODO to README.md

[alexgreenbank]

Minor tweak to note which packages need consolidation and obtaining from source rather than being duplicated in this repo.

[grafanabot]

Go coverage report:

Click to expand.

File	%
github.com/grafana/influx2cortex/pkg/influx/api.go	93.3%
github.com/grafana/influx2cortex/pkg/influx/errors.go	100.0%
github.com/grafana/influx2cortex/pkg/influx/parser.go	84.7%
github.com/grafana/influx2cortex/pkg/influx/proxy.go	61.9%
github.com/grafana/influx2cortex/pkg/influx/recorder.go	91.7%
github.com/grafana/influx2cortex/pkg/tenant/resolver.go	75.6%
github.com/grafana/influx2cortex/pkg/tenant/tenant.go	91.7%
github.com/grafana/influx2cortex/pkg/util/extract_forwarded.go	66.7%
github.com/grafana/influx2cortex/pkg/util/http.go	0.0%
github.com/grafana/influx2cortex/pkg/util/log/log.go	0.0%
github.com/grafana/influx2cortex/pkg/util/log/rate_limit.go	90.0%
github.com/grafana/influx2cortex/pkg/util/log/wrappers.go	0.0%
total	56.6%

Go lint report:

No issues found. 😎