Add a CONTRIBUTING.md file

CONTRIBUTING.md

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+# Getting a .war
+
+Airsonic is using [Maven](https://maven.apache.org/) to manage its build
+process. Any version above 3.3+ should do the job.
+
+If you want to run the testsuite and get a `.war` is everything went fine,
+you this command:
+
+```
+$ mvn clean package 
+```
+
+If you don't care about the result of the testsuite, but only
+want a `.war` as quick as possible, you can use this instead:
+
+```
+$ mvn -Dmaven.test.skip=true clean package 
+```
+
+# Suggesting modifications
+
+Airsonic's source code is hosted on [github](https://github.com/airsonic/airsonic/),
+who provides a [lot of documentation](https://help.github.com/en) on how
+to contribute to projects hosted there.
+
+# Getting help
+
+The documentation is hosted [here](https://airsonic.github.io/) (you can
+contribute to it [here](https://github.com/airsonic/documentation)), and aims
+at being comprehensive. You can also use [irc](irc://irc.freenode.net/airsonic)
+and [reddit](https://www.reddit.com/r/airsonic/) if you want to discuss or ask
+questions.