Create a helm chart style guide

[mattmceuen]

This issue is twofold:

First, to lay the groundwork for a helm chart style guide, as a .md document in the airship charts project. Over time this can grow into a comprehensive set of recommendations for Airship's opinions around when to use charts, where charts should live, how charts should be written, and how they should behave.

Second, to specifically add content around one pressing concern: we need to make sure that any CRDs that are applied via a helm chart are not deleted when the chart is deleted. Otherwise, we run the risk of impacting running workloads during operational activities. Helm v2 and v3 have different mechanisms to solve for this problem, and this style guide should document (and/or link to documentation for) both:

• for Helm v2, there is a hook that allows CRDs to be deployed separately from resources
• for Helm v3, there is a crds/ folder that allows CRDs to be deployed separately from resources

Any related information that would be helpful to chart developers should be included in this guide.

[mattmceuen]

Some helpful pointers from @seaneagan:

Helm CRD Handling:
Helm 2: https://v2.helm.sh/docs/chart_best_practices/#method-2-crd-install-hooks
Helm 3: https://helm.sh/docs/chart_best_practices/custom_resource_definitions/#method-1-let-helm-do-it-for-you
Problem description: helm/community#138