Update 'README.md'

README.md

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 This repository contains data and code for our 2020 manuscript *"Gaze stabilisation behaviour is anisotropic across visual field locations in zebrafish"* by Dehmelt, Meier, Hinz et al. The corresponding author is [Aristides B. Arrenberg](mailto:aristides.arrenberg@uni-tuebingen.de).
 
+#### Prerequisites:
+
+MATLAB versions 2015a to 2020a should work. The code recreating the figures requires the open-source "cbrewer" package published by Cynthia Brewer, e.g. [via MATLAB Central](https://de.mathworks.com/matlabcentral/fileexchange/34087-cbrewer-colorbrewer-schemes-for-matlab).
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 #### The "data" and "code" folders:
 
 These contain pre-processed data in MAT files, and MATLAB code to reproduce all or parts of Figures 3-6, as well as Figure 3-figure supplement 5. Data file paths are relative to the code file paths, and are hard-coded as such in the M files; so *the "data" and "code" folders should always be placed in the same location*. If you want to download only some of the subfolders (e.g., only those pertaining to a single figure), make sure to nonetheless place them in two neighbouring folders called "data" and "code".