rescribe: add documentation on how to generate embedded data

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+The embedded copies of Tesseract are fetched by `go generate` from
+copies that are stored online.
+
+To create them yourself, you need to create a .zip file that contains
+the tesseract executable, plus any libraries that are needed for it
+to run.
+
+It must be linked so that these libraries are accessed from the same
+directory as the executable. On Windows this is the default
+behaviour. On Linux we just create a static binary using the
+simplemake branch of Tesseract, available at
+https://github.com/nickjwhite/tesseract
+
+On OSX it's a bit more complicated. We install Tesseract on a host
+machine using homebrew, then copy the binary, and run
+`otool -L tesseract` to find the libraries that need to be copied
+as well. Then `otool -L libname.dylib` needs to be run for each
+library to find all non-system libraries they depend on, to copy.
+Once that is done, `install_name_tool` needs to be run on the
+binary and libraries to set the lookup path to the local directory,
+like this:
+  install_name_tool -change /usr/local/opt/libpng/lib/libpng16.16.dylib @executable_path/libpng16.16.dylib liblept.5.dylib
+You can find the path names to replace using `otool -L`.
+This is all taken from a great guide on how to do this:
+http://thecourtsofchaos.com/2013/09/16/how-to-copy-and-relink-binaries-on-osx/
+
+The embedded tessdata is much easier to create, it's just a
+standard tessdata from an install on any platform, plus any
+additional .traineddata files you want to include.