Buildpack API 0.6

This release of the Cloud Native Buildpacks Specification defines Buildpack API 0.6.

Additions

• Buildpacks may contribute the default process type (#159, #205, RFC 0064).
• Adds exec.d support on Windows (#164, #203).
• Adds new descriptor keys to buildpack.toml (#181, #208, RFC 0070).

Breaking Changes

• Buildpacks must explicitly opt-in to layer re-use by appending to <layers>/<layer>.toml (#132, #209, RFC 0052).
• Adds types table to <layers>/<layer>.toml (#185, #209, RFC 0074).

Nonfunctional Changes

• Explicitly bans the use of special layer names by buildpacks (#202)
• Clarifies the role of the launcher in setting environment variables at runtime (#207)
• Fixes links to exec.d interface (#168)