Document alternative modes of using the buildpack

There is no documentation about the different ways that the buildpack can be
used. This commit add documentation about expert mode and offline buildpacks.

[#76619826]
Further changes to expert mode documentation

These changes bring the submission in line with the suggestions in
issue #96.

[#76619826]
Merge branch 'cgfrost-76619826-document-expert-mode'

[resolves #98]