Add brief description to the top of .api dumps, to help explain the file and BCV

[aSemy]

Currently the .api dumps have no description. Their purpose is unclear to anyone not familiar with BCV, and there's no easy way to discover that they are created by BCV.

I suggest adding a brief comment to the top of all .api files, covering some basic information:

# Kotlin Binary Compatibility Validator dump file.
# This file is automatically generated, do not modify.
# For more information, see https://github.com/Kotlin/binary-compatibility-validator

[the remaining dump file...]

(I've assumed that BCV would implement a limited comment feature, so BCV would ignore lines at the start of the file that start with #.)