Add documentation for the recently merged ypkg `%symlink_check` macro

[Rune Morling]

Ypkg PR #62 reads:

This adds the %symlink_check macro to look for and exit on finding broken symlinks.

Exporting SYMLINK_WARN will make it just print the broken symlinks and warning message without halting the package build.

The comment in the macro says:

"Check for broken symlinks, turn from error into warning by setting SYMLINK_WARN"

It is apparently used to help make NVIDIA driver kernel builds less brittle?

The PR itself mentions the draft PR Nvidia symlink safeguard.