Merge pull request #808 from ulb-sachsen-anhalt/doc/upd/install-deps

[doc][upd] note install requirements

Author: kba

README.md

@@ -63,6 +63,10 @@ pip install ocrd_modelfactory
 
 All python software released by [OCR-D](https://github.com/OCR-D) requires Python 3.6 or higher.
 
+**NOTE** Some OCR-D-Tools (or even test cases) _might_ reveal an unintended behavior if you have specific enviroment modifications, like:
+* using a custom build of [ImageMagick](https://github.com/ImageMagick/ImageMagick), whose format delegates are different from what OCR-D supposes
+* custom Python logging configurations in your personal account
+
 ## Command line tools
 
 **NOTE:** All OCR-D CLI tools support a `--help` flag which shows usage and