Installation instructions

INSTALL.md

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+All programs require the installation of Python 3, which can be
+obtained at <https://www.python.org/downloads/> or through package
+managers (depending on the operating system). Current installations
+already include `pip` which is used to install further packages.
+
+In a terminal, run the command
+
+    pip3 install lxml bibtexparser bs4
+
+to install the required packages.
+
+In order to run the xsl scripts found in the `data` directory, an XSL
+processor like [Saxon](http://saxon.sourceforge.net/) is required.