Add README for ComCam transmission_sensor data.

comCam/transmission_sensor/README.md

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+This directory contains baseline sensor QE curves for the LSSTComCam instrument.
+The LSSTComCam focal plane consists of one raft of ITL sensors.
+As we do not have QE scans of LSSTComCam, we have simply taken an average of all the ITL sensors from the LSSTCam sensor QE scans as processed in DM-40164.
+
+Data Processing
+---------------
+
+To generate these files, you run this script from obs_lsst.
+```
+python python/lsst/obs/lsst/script/write_comcam_estimated_transmission_sensor.py
+```