readme and documentation updates

docs/docker.md

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+# 1. ocrd resource manager
+(just once, to get the models and install them into a named volume for later re-use)
+
+    vol_models=ocrd-resources:/usr/local/share/ocrd-resources
+    docker run --rm -v $vol_models ocrd/eynollah ocrd resmgr download ocrd-eynollah-segment default
+
+Now, each time you want to use Eynollah, pass the same resources volume again.
+Also, bind-mount some data directory, e.g. current working directory $PWD (/data is default working directory in the container).
+Either use standalone CLI (2) or OCR-D CLI (3):
+
+# 2. standalone CLI (follow self-help, cf. readme)
+
+    docker run --rm -v $vol_models -v $PWD:/data ocrd/eynollah eynollah binarization --help
+    docker run --rm -v $vol_models -v $PWD:/data ocrd/eynollah eynollah layout --help
+    docker run --rm -v $vol_models -v $PWD:/data ocrd/eynollah eynollah ocr --help
+
+# 3. OCR-D CLI (follow self-help, cf. readme and https://ocr-d.de/en/spec/cli)
+
+    docker run --rm -v $vol_models -v $PWD:/data ocrd/eynollah ocrd-eynollah-segment -h
+    docker run --rm -v $vol_models -v $PWD:/data ocrd/eynollah ocrd-sbb-binarize -h
+
+Alternatively, just "log in" to the container once and use the commands there:
+
+    docker run --rm -v $vol_models -v $PWD:/data -it ocrd/eynollah bash

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