Add main calibrations directory

(cherry picked from commit c7fc035b)

calibrations/README.md

+## Calibrations for the ATHENA detector
+
+Calibrations and constants for this ATHENA detector setup. These values are used in the
+reconstruction. Each relevant subsystem that requires calibration data should have
+its own subdirectory with relevant calibration files, preferably in JSON format.
+
+Note that these calibrations are strongly versioned and tied directy to this specific
+detector configuration. Therefore, please do not add an extra layer of versioning in the
+file names, but rather only add those calibrations relevant for this particular setup.