add python library requirements

requirements.txt

@@ -0,0 +1,13 @@
+# Requirements for dicom_indexer.py and dicom_sender.py
+#
+# Install with:
+#   pip install -r requirements.txt
+#
+# pydicom    - required by both scripts (reading DICOM metadata/files)
+# openpyxl   - required only if using dicom_indexer.py's --xlsx option,
+#              or dicom_sender.py reading a .xlsx index file
+# pynetdicom - required only by dicom_sender.py (C-ECHO / C-STORE network operations)
+
+pydicom>=3.0.2
+openpyxl>=3.1.5
+pynetdicom>=3.0.4