Update ADD_NEW_DATA.md

Author: jfy133

docs/ADD_NEW_DATA.md

@@ -6,6 +6,7 @@ Please fill in the appropriate checklist below (delete whatever is not relevant)
    - If this is the case, follow the [documentation on how to use an existing test dataset](https://github.com/nf-core/test-datasets/blob/master/docs/USE_EXISTING_DATA.md)
  - [ ] Fork the [nf-core/test-datasets repository](https://github.com/nf-core/test-datasets) to your GitHub account
  - [ ] Create a new branch on your fork
+ - [ ] Check your proposed test data follows the [guidelines](https://nf-co.re/docs/contributing/test_data_guidelines)
  - [ ] Add your test dataset
    - [ ] If you clone it locally use `git clone <url> --branch <branch> --single-branch`
  - [ ] Make a PR on a new branch with a relevant name