Add CONTRIBUTING file

CONTRIBUTING.md

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+# Contributing
+
+You can send contributions and comments to the [OpenRail Association web site](https://openrailassociation.org) by creating pull requests or opening issues in this repository.
+
+The site is built with [Hugo](https://gohugo.io/). If you want to check the rendering of the web site locally, you can install Hugo and run it in the local check out of the git repository:
+
+    hugo server
+
+You can then view the site with your web browser by going to the address the command tells you.
+
+If you change existing content you might not need to use the local preview, but if you are changing the structure or mechanics of the site, it is recommended to check that with the local preview.