Islandora IIIF: Add instructions for setting up a context reaction fo…

…r the IIIF Dimensions actions.

modules/islandora_iiif/README.md

@@ -58,6 +58,23 @@ To use it, either:
 - Add it as a derivative reaction to a node with an Original FIle as its child, or
 - Use it as a batch action, such as on a Paged Content object's list of child pages.
 
+### Setting up the action as a Context Reaction
+
+These instructions assume a standard Islandora Starter Site, so if you have different field names or Contexts, adjust accordingly.
+
+1. Go to admin/config/system/actions
+2. Choose "Add image dimensions retrieved from the IIIF server" from the Create Complex Action drop-down and then click Create.
+3. Enter Original File for the Source Media Use term.
+4. Choose media -- file -- Width and Media -- File -- Height for the corresponding configuration values and click Save.
+5. Go to admin/structure/context, and click Duplicate oon the Page Derivatives row.
+6. Name the new context something like "Retrieve Page Dimensionss". and edit it.
+7. This is the tricky bit, delete 'Original File' from the 'Media has term with URI' field and replace it with Service File. The explanation for this is that to retrieve a file from the IIIF server, it must be part of an Islandora Media that has been fully created and saved and given a URL. This hasn't happened yet when Original File derivatives are being created, so we need to hang our action onto a derivative that is created after the original one.
+8. Under Reactions, deselect the existing actions and select  "Add i mage dimensions from IIIF sserver" and click Save and continue.
+9. Go back to your Paged Content object and add another child with a File media, to which you should upload another TIFF or JP2 file.
+10. Without going  to the Original File Book Manifestt, make sure that a service file has been generated, then click Edit on the Original File media.
+11. Ensure that the Width and Height fields are populated with thee correct values from the file.
+
+
 ## Documentation
 
 Official documentation is available on the [Islandora 2 documentation site](https://islandora.github.io/documentation/).