Added documentation

docs/cms-translation-plugin-setup.md

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+# CMS translation plugin setup
+
+## Add plugin to DatoCMS project
+
+- Go to the DatoCMS project.
+- Go to configuration > Plugins.
+- Click the `Add a new plugin` button on the bottom of the panel.
+- Click `From Marketplace`.
+- Search for `datocms-plugin-translate-fields`.
+- Open it and click `Install plugin!`.
+
+## Apply to fields
+
+- Turn on `Auto apply to fields`.
+
+## Add a translation service
+
+- Choose a translation service.
+- Add the API key.
+- (optional) Fill in additional fields.