Update Docs: Account “external-oauth-clients” (#118)

Add OAuth client implementation for Google Apps Script

Author: nuzulul

content/docs/external-oauth-clients.md

@@ -26,5 +26,6 @@ If you would like to facilitate your own OAuth authentication rather than use Ne
 | [@alukovenko](https://github.com/alukovenko/)                | GitHub                            | Go                                        | [Repo](https://github.com/alukovenko/decapcms-oauth2)                                                                                                          |
 | [@BTx123](https://github.com/BTx123)                         | GitHub, GitHub Enterprise, GitLab | Firebase V2 Cloud Function                | [Repo](https://github.com/BTx123/decap-cms-oauth-firebase)                                                                                                     |
 | [@i40west](https://github.com/i40west)                       | GitHub                            | Cloudflare Pages                          | [Repo](https://github.com/i40west/netlify-cms-cloudflare-pages)                                                                                                |
+| [@nuzulul](https://github.com/nuzulul)                       | GitHub                            | Google Apps Script                        | [Repo](https://github.com/nuzulul/decap-cms-google-apps-script)                                                                                                |
 
 Check each project's documentation for instructions on installation and usage.