Document the bin/pre_compile and bin/post_compile hooks

[edmorley]

The buildpack supports running arbitrary scripts at the start and end of the compile, by creating a bin/pre_compile and/or bin/post_compile file in the app's source directory.

However this feature is not currently documented anywhere.

GUS-W-7918494.

[git2gus]

This issue has been linked to a new work item: W-7918494