If you're about to file an issue for some suggested change, please consider instead making the change yourself and submitting it as a pull request. Pull requests are much more likely to be quickly acted on, so they're the best way to get your change into the masterlist.
LOOT's masterlists are versioned. Before making any changes, please make sure that you're working on the correct branch for LOOT's latest release.
If you are new to contributing to projects on GitHub, the How To Contribute wiki page is a good starting point.
The format and syntax of the masterlist is fully documented (an offline copy is also available in the docs folder of each LOOT release), and other information on how to edit the masterlist can be found on the Masterlist Editing wiki page.
If you send a pull request, then discover that you have made a mistake and make additional commits to fix it, please squash those additional commits to keep the commit history tidy. If you squash your commits, you don't need to open a new pull request. If you don't know how to squash commits, whoever merges your pull request can do it for you.
Pull requests and all commits to the masterlist repository are automatically run through a validator to check for syntax errors.
Nevertheless, it is advisable to test your changes on your own LOOT install before sending a pull request, as they may have valid syntax but not have the intended effect. Instructions on how to do so can be found on LOOT's wiki.