Add event-submitter-polyfill note

Safari does not support submitters on submit events. This may be needed
for checking which button was clicked to submit forms.

README.md

@@ -161,6 +161,12 @@ needed, the proper usages are documented via TSDoc comments.
 The library has been tested on the static DOM using JSDOM, and the real dynamic DOM using Cypress. This is to guarantee
 compatibility across environments.
 
-See the [documentation folder](https://code.modal.sh/TheoryOfNekomata/formxtra/src/branch/master/docs) for more details on this library.
+See the [documentation folder](https://code.modal.sh/TheoryOfNekomata/formxtra/src/branch/master/docs) for more details
+on this library.
+
+You may need the [event-submitter-polyfill package](https://github.com/idea2app/event-submitter-polyfill) for browsers
+that do not support the submitter property on submit events (such as Safari). The submitter element is needed for
+tracking which button triggered a form submission, and it may contain `name` and `value` attributes which are used to
+add values to the form (such as specifying which action to take for the rest of the form values).
 
 The sources are under the [MIT license](https://code.modal.sh/TheoryOfNekomata/formxtra/raw/branch/master/LICENSE).