Add a guide for (new) contributors

CONTRIBUTING.md

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+# Contributing to the CartoCSS map styles of OpenRailwayMap
+
+In this document, we provide a few tips if you want to contribute to the OpenRailwayMap CartoCSS map styles by creating pull requests.
+
+## Code formatting
+
+We use two spaces for indentation in our MapCSS and YAML files.
+
+## Test your code
+
+We like pull requests with example renderings. We will still make our own test renderings
+but your test renderings help you to find bugs. You can use the provided Docker setup.
+
+If you provide test renderings or not, we appreciate recommendations for locations where to
+make our test renderings.
+
+## New signalling systems
+
+Each country (sometimes each company) has their own signalling system. We welcome
+other systems than those already supported by the style. New signals must be documented
+on the OSM wiki and be actively used.
+
+## Legend
+
+The legend at the OpenRailwayMap website explains users what the colors and
+symbols on the map mean. It still makes use of the
+[old MapCSS styles](https://github.com/OpenRailwayMap/OpenRailwayMap/tree/master/styles)
+in the main repository. The `.mapcss` files are the style, the `.json` files
+are the definition of the map key.
+
+## Icons
+
+All icons must be SVG files because we do not want to leave the users of the
+style the freedom to render map images in any resolution they want. If someone
+wants to print out OpenRailwayMap, they will benefit from a sharp icons.
+
+Please mind the following recommendations when you add new icons:
+
+* Country specific icons should be saved in `symbols/<country_code>/`.
+  If your country has different signals for different railway networks (e.g.
+  heavy rail and trams), it will be a good idea to create a subdirectory within
+  the country's directory. See Germany as an example.
+* File names should match either the signal names, their abbreviations or their
+  English names.
+* Keep in mind that the icons will be pretty small. Don't draw to detailed graphics.
+* Icons should not have any padding.
+* Signals (including boards) should not have a pole (except corner cases like poles
+  being a signal).
+* SVG filters are not supported by Mapnik.
+* Text should be inserted as paths.
+* Please have a look at the XML code of your SVG files. We do not want XML code
+  full of metadata from your drawing software.
+* We do not remove unnecessary whitespace from our SVG files.
+
+## Icons for speed signals
+
+Icons for speed signals should be provided in all existing variants if the speed signal
+displays a number. If the signal shows a 5 for 50 kph, we want to render the icon with a 5,
+not 9. Do not worry if you do not know all variants of that icon. Just add some variants,
+the other ones can be add in the future.
+
+## Other remarks
+
+Please do not mix pure formatting changes with other changes. Diffs are read by humans. ;-)