Document abstract outline of package in fluxcd/pkg/runtime directory

[scottrigby]

Follow up to #117

This is a top level README in fluxcd/pkg/runtime.

This should be a markdown version in the package as a GitHub landing page, which outlines and glues together the in-depth directory contents, but doesn’t give any advice on how to use them (that will be added separately to the website). To show users code examples, link to inline code examples in the go documentation. The idea is for this to be a lighter version of what we will put on the website, in the context of runtime package only.

Contains:

• Description (in go doc directory)
• A more human description of what’s written in the package descriptions
• Intro
• List of things that are in there
• Generic list of subjects (standards, or other things to take into account if you make use of the runtime package or contributing to Flux)
• When we mention our standards and API conventions, we can link to the packages that help with those standards

/assign @hiddeco