add Plotly.setPlotConfig step in plotly.js-locales README

Author: etpinard

tasks/sync_packages.js

@@ -205,6 +205,7 @@ function syncLocalesPkg(d) {
             '',
             '// then',
             'Plotly.register(locale);',
+            'Plotly.setPlotConfig({locale: \'fr\'})',
             '```',
             '',
             copyrightAndLicense