Added README file for Spitzer images

data/spitzer_images/README.md

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+# *Spitzer* IRAC1 (3.6 micron) Images (data, mask, PSF)
+
+The data images have been converted to integrated counts (ADU) per pixel.
+
+The NGC 4608 images are based on the pipeline-generated MAIC mosaic images
+from the [*Spitzer* Legacy Archive](http://sha.ipac.caltech.edu/applications/Spitzer/SHA/), 
+with a pixel scale of 0.6 arcsec.
+
+The NGC 4643 images are based on the [S4G-generated mosaic images](https://irsa.ipac.caltech.edu/data/SPITZER/S4G/), 
+with a pixel scale of 0.75 arcsec and standard (N = up, E = left) astronomical
+orientation.
+
+The PSF images are based on the [in-flight Point Response Funtion files](https://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/calibrationfiles/psfprf/),
+downsampled to the appropriate pixel scales.