Add documentation for creating multi-platform builders

[jjbustamante]

Summary

Pack 0.34.0 is expected to be released very soon, in this new version we are implementing the RFC 0128 which enables the capability to create multi-platform builders, we need to update our documentation with the important guidelines for Buildpack Authors to take advantage of this new feature

What questions should this documentation answer?

• We should answer:
  • How can we create multi-platform builders?
  • What are the current limitations of the feature?

Who is the target audience for this documentation?

• App Developer
• Buildpack Author
• Operator
• Other:

Additional Information