DISCOURSE_ADDITIONAL_PLUGINS description in Readme

README.md

 # docker-discourse
+
 discourse image for discourse service
+
+## Discourse plugins
+
+This image supports installing Discourse plugins at build time, via the `DISCOURSE_ADDITIONAL_PLUGINS` [build arg](https://docs.docker.com/engine/reference/builder/#/arg). Set it to a whitespace (space, tab, newline) separated list if valid `git` URLs of plugins to be installed at build time.
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