Extra help in readme

README.md

@@ -2,7 +2,7 @@
 
 Align a sequence of astronomical FITS files based on sources extracted in each image. The aligned files will be geometrically reprojected so all the images are the same size and shape.
 
-It is designed to work exclusively with reduced data from [Las Cumbres Observatory](https://lco.global). FITS data from LCO is [Rice compressed](https://heasarc.gsfc.nasa.gov/fitsio/fpack/) and contains sources extracted using [SEP](https://sep.readthedocs.io/en/v1.0.x/) during our data pipeline processing in a catalog (`CAT`) FITS Header Data Unit (HDU).
+It is designed to work exclusively with reduced data from [Las Cumbres Observatory](https://lco.global). FITS data from LCO is [Rice compressed](https://heasarc.gsfc.nasa.gov/fitsio/fpack/) and contains sources extracted using [SEP](https://sep.readthedocs.io/en/v1.0.x/) during our data pipeline processing in a catalogue (`CAT`) FITS Header Data Unit (HDU).
 
 ## Installation
 
@@ -53,6 +53,14 @@ for i, catfile in enumerate(img_list):
         catalogues.append(data)
 ```
 
+## HDU containing catalogues
+
+If you are using a none standard place for your catalogue HDU, that can be passed in a parameter to the `make_transforms()` function i.e.
+
+```
+identifications = make_transforms(ref_image, images_to_align, hdu=0)
+```
+
 ## About
 
 This is a customised fork of [AliPy by Malte Tewes](http://obswww.unige.ch/~tewes/alipy/). I have removed the dependency on SciPy in favour of pure NumPy (for linear algebra) and Pillow (for image array transforms).