User's dont understand why we select workloads using labels.

[squeedee]

Description of problem

User's have reported that

1. It's not obvious that a particular label is needed to select the correct supply chain
2. confusion that we do not just have a 'supplyChainRef' in the workload.

Proposed solution

Explain in the docs how the label selectors work AND explain why that approach is used.

[squeedee]

How: https://cartographer.sh/docs/v0.2.0/architecture/#selectors But still no why