Document mixing and matching better.

[jonhoo]

See this Reddit answer for example. Pointing out how users can do things like

 - stage: feature-check
   jobs:
    - job: feature-check
      displayName: cargo check --features XYZ
      pool:
        vmImage: ubuntu-16.04
      steps:
       - template: azure/install-rust.yml@templates
       - script: cargo check --features xyz
         displayName: Run cargo check w/ XYZ

Somewhere that's easy to find is probably a good idea! This will likely go along with finding a better way to structure the README.