document how to use the local_spectrum field

Let's say full local path is /Users/john/Science/BYW_spectra/FINALSAMPLESPECTRA/.

You can use a system variable so that any user could have a local version.
$BYW_spectra = "/Users/john/Science/BYW_spectra/FINALSAMPLESPECTRA"

local_spectrum = "$BYW_spectra/"

Locate the documentation or schema entry for the local_spectrum field; no file or test is named in the issue. Use the supplied BYW_spectra environment-variable example to explain replacing the absolute path, and consider the work done when users can configure a local spectrum path without editing the example path directly.

Written by the indexing model from the issue text.