doc: briefly explain what's included

PR-URL: npm#11188
Credit: @beaugunderson
Reviewed-By: @othiym23

doc/cli/npm-publish.md

@@ -11,9 +11,12 @@ npm-publish(1) -- Publish a package
 
 ## DESCRIPTION
 
-Publishes a package to the registry so that it can be installed by name. See
-`npm-developers(7)` for details on what's included in the published package, as
-well as details on how the package is built.
+Publishes a package to the registry so that it can be installed by name. All
+files in the package directory are included if no local `.gitignore` or
+`.npmignore` file exists. If both files exist and a file is ignored by
+`.gitignore` but not by `.npmignore` then it will be included.  See
+`npm-developers(7)` for full details on what's included in the published
+package, as well as details on how the package is built.
 
 By default npm will publish to the public registry. This can be overridden by
 specifying a different default registry or using a `npm-scope(7)` in the name