readme and documentation updates

docs/ocrd.md

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+When using Eynollah in OCR-D, the source image file group with (preferably) RGB images should be used as input like this:
+
+    ocrd-eynollah-segment -I OCR-D-IMG -O OCR-D-SEG -P models eynollah_layout_v0_5_0
+
+If the input file group is PAGE-XML (from a previous OCR-D workflow step), Eynollah behaves as follows:
+- existing regions are kept and ignored (i.e. in effect they might overlap segments from Eynollah results)
+- existing annotation (and respective `AlternativeImage`s) are partially _ignored_:
+  - previous page frame detection (`cropped` images)
+  - previous derotation (`deskewed` images)
+  - previous thresholding (`binarized` images)
+- if the page-level image nevertheless deviates from the original (`@imageFilename`)
+  (because some other preprocessing step was in effect like `denoised`), then
+  the output PAGE-XML will be based on that as new top-level (`@imageFilename`)
+
+      ocrd-eynollah-segment -I OCR-D-XYZ -O OCR-D-SEG -P models eynollah_layout_v0_5_0
+
+In general, it makes more sense to add other workflow steps **after** Eynollah.
+
+There is also an OCR-D processor for binarization:
+
+    ocrd-sbb-binarize -I OCR-D-IMG -O OCR-D-BIN -P models default-2021-03-09