Update README with usage and requirements

Added usage instructions and requirements for the converter.

README.md

@@ -1,2 +1,16 @@
 # XLSX-converter-to-EML
 Excel to Election Markup Language converter
+
+Converts polling station data from Excel (.xlsx) to EML_NL 110b XML format.
+
+Usage:
+
+  1. Edit config.json with your election settings
+  2. Run: python xlsx_to_eml.py
+  3. Select your Excel file in the file dialog
+  4. Choose where to save the output EML.xml file
+
+Requirements:
+```python
+pip install openpyxl
+```