Mention requirements

tikz/gpx-gps-elevation-chart-generator/README.md

@@ -3,5 +3,16 @@ Compiled example
 ![Example](elevation-chart.png)
 
 
+## Requirements
+
+You need `gpxpy`. Install it via
+
+```bash
+$ pip install gpxpy --user
+```
+
+
+## Run
+
 Run `python elevation_chart_gen.py -f ../Downloads/Jul_3,_2016_12_52_51.gpx` to
 generate the `data.csv`. Then run `make` to generate the chart.