Document Layer Reuse criteria

[fg-j]

What happened?

• What were you attempting to do?
  I wanted to quickly understand what criteria the yarn-install buildpack uses to decide if it'll reuse the cached node_modules layer.

• What did you expect to happen?
  I expected to find this documented either in the README for the buildpack or somewhere in the Nodejs section of the Docs site.

• What was the actual behavior? Please provide log output, if possible.
  There's no documentation that I could find, anywhere, of the layer reuse criteria for any of the buildpacks.

Checklist

• I have included log output.
• The log output includes an error message.
• I have included steps for reproduction.

[fg-j]

@ekcasey Is this documented for any of the Java buildpacks? Do you think this should be documented? If so, where? (I recall you hoping for some sort of Paketo buildpacks reference docs in the future.)

[kvedurmu]

@fg-j since it seems like this is a Node.js specific request, could we move this issue into a Node.js repo so that one of the maintainers can get visibility?

[fg-j]

cc @arjun024 @thitch97

@kvedurmu I'd rather leave it here since, ultimately, the markdown for these docs live in this repo (for now).

[ekcasey]

@fg-j this isn't currently documented for the Java buildpacks. I agree that this level of detail would be nice to include in future reference docs

cc @dmikusa-pivotal