Initial commit: Developer Tools MVP with visual editor

- Complete React app with 7 developer tools
- JSON Tool with visual structured editor
- Serialize Tool with visual structured editor
- URL, Base64, CSV/JSON, Beautifier, Diff tools
- Responsive navigation with dropdown menu
- Dark/light mode toggle
- Mobile-responsive design with sticky header
- All tools working with copy/paste functionality

node_modules/bfj/CONTRIBUTING.md

@@ -0,0 +1,37 @@
+# Contribution guidelines
+
+* Install/update the dependencies:
+  ```
+  npm install
+  ```
+
+* Make sure the code lints:
+  ```
+  npm run lint
+  ```
+
+* Make sure all of the tests pass:
+  ```
+  npm test
+  ```
+
+* Adhere to the coding conventions
+  that are used elsewhere in the codebase.
+
+* New code must have new unit tests.
+
+* New features
+  or changes to existing features
+  must be documented in the [readme] file.
+
+* Add yourself to the [authors] file.
+
+* Feel free to [open an issue][newissue] first,
+  if the change is one that you think
+  needs some discussion.
+
+[readme]: https://gitlab.com/philbooth/bfj/blob/master/README.md
+[authors]: https://gitlab.com/philbooth/bfj/blob/master/AUTHORS
+[newissue]: https://gitlab.com/philbooth/bfj/issues/new
+[issues]: https://gitlab.com/philbooth/bfj/issues
+