add developers.md

Author: ufechner7

docs/developers.md

@@ -0,0 +1,19 @@
+## Notes for developers
+
+### How to check the .cff file
+
+You can validate the `CITATION.cff` file with the command:
+```bash
+cffconvert --validate
+```
+
+You can install the script `cffconvert` with the command:
+```bash
+python3 -m venv venv
+source venv/bin/activate
+python3 -m pip install cffconvert
+```
+After using the tool, deactivate the Python environment with:
+```bash
+deactivate
+```