Update i18n module doc with Plugin development section

docs/modules/I18n.md

@@ -139,4 +139,81 @@ We need to find the way to update the UI
 
 ## Plugin development
 
-If you're developing a plugin for GrapesJS and you need to support some string localization or simply...
+::: warning
+This section is dedicated **only** to plugin developers and can also be skipped in case you use [grapesjs-cli](https://github.com/artf/grapesjs-cli) to init your plugin project
+:::
+
+If you're developing a plugin for GrapesJS and you need to support some string localization or simply change the default one, we recommend the following structure.
+
+```
+plugin-dir
+- package.json
+- README.md
+- ...
+- src
+    - index.js
+    - locale // create the locale foldar in your src
+        - en.js // All default strings should be placed here
+```
+
+For your plugin specific strings, place them under the plugin name key
+
+```js
+// src/locale/en.js
+export default {
+    'grapesjs-plugin-name': {
+        yourKey: 'Your value',
+    }
+}
+```
+
+In your `index.js` use the `en.js` file and add `i18n` option to allow import of other local files
+
+```js
+// src/index.js
+import en from 'locale/en';
+
+export default (editor, opts = {}) => {
+    const options = {
+        i18n: {},
+        // ...
+        ...opts,
+    };
+
+    // ...
+
+    editor.I18n.addMessages({
+        en,
+        ...i18n,
+    });
+}
+```
+
+The next step would be to compile your locale files into `<rootDir>/locale` directory to make easily accessible by your users. This folder could be ignored in your git repository be should be deployd to the npm registry
+
+::: warning
+Remember that you can skip all these long steps and init your project with [grapesjs-cli](https://github.com/artf/grapesjs-cli). This will create all the necessary folders/files/commands for you (during `init` command this step is flagged `true` by default and we recommend to keep it even in case the i18n is not required in your project)
+:::
+
+
+At the end, your plugin users will be able to import other local files (if they exist) in this way
+
+```js
+import grapesjs from 'grapesjs';
+
+// Import from your plugin
+import yourPlugin from 'grapesjs-your-plugin';
+import ch from 'grapesjs-your-plugin/locale/ch';
+import fr from 'grapesjs-your-plugin/locale/fr';
+
+const editor = grapesjs.init({
+  ...
+  plugins: [ yourPlugin ],
+  pluginsOpts: {
+      [yourPlugin]: {
+          i18n: { ch, fr }
+      }
+  }
+});
+```
+