Add CONTRIBUTING.md (closes #76)

CONTRIBUTING.md

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+# Contributor guide
+
+The Scala Maxmind IpLookups library is maintained by the pipeline team at Snowplow Analytics and improved on by
+external contributors for which we are extremely grateful.
+
+## Getting in touch
+
+### Community support requests
+
+First and foremost, please do not log an issue if you are asking a general question, all of our community support requests
+go through our Discourse forum: https://discourse.snowplowanalytics.com/.
+
+Posting your problem there ensures more people will see it and you should get support faster than creating a new issue on
+GitHub. Please do create a new issue on GitHub if you think you've found a bug or if you would like to submit a feature
+request though!
+
+### Gitter
+
+If you want to discuss already created issues, potential bugs, new features you would like to work on or any kind of developer
+chat, you can head over to our [Gitter room](https://gitter.im/snowplow/scala-maxmind-iplookups).
+
+## Issues
+
+### Creating an issue
+
+The project contains an issue template which should help guiding you through the process. However, please keep in mind
+that support requests should go to our Discourse forum: https://discourse.snowplowanalytics.com/ and not GitHub issues.
+
+It's also a good idea to log an issue before starting to work on a pull request to discuss it with the maintainers.
+
+### Working on an issue
+
+If you see an issue you would like to work on, please let us know in the issue! That will help us in terms of scheduling and
+not doubling the amount of work.
+
+If you don't know where to start contributing, you can look at
+[the issues labeled `good first issue`](https://github.com/snowplow/scala-maxmind-iplookups/labels/good%20first%20issue).
+
+## Pull requests
+
+These are a few guidelines to keep in mind when opening pull requests, there is a GitHub template that reiterates most of the
+points described here.
+
+### Commit hygiene
+
+We keep a strict 1-to-1 correspondance between commits and issues, as such our commit messages are formatted in the following
+fashion:
+
+`Add issues description (closes #1234)`
+
+for example:
+
+`Introduce an error ADT (closes #1234)`
+
+### Writing tests and running tests
+
+Whenever necessary, it's good practice to add the corresponding unit tests to whichever feature you are working on.
+
+Then you can run `sbt test` to check they are working properly.
+
+### Feedback cycle
+
+Reviews should happen fairly quickly during weekdays. If you feel your pull request has been forgotten, please ping one
+or more maintainers in the pull request.
+
+### Getting your pull request merged
+
+If your pull request is fairly chunky, there might be a non-trivial delay between the moment the pull request is approved and
+the moment it gets merged. This is because your pull request will have been scheduled for a specific milestone which might or
+might not be actively worked on by a maintainer at the moment.
+
+### Contributor license agreement
+
+We require outside contributors to sign a Contributor license agreement (or CLA) before we can merge their pull requests.
+You can find more information on the topic in [the dedicated wiki page](https://github.com/snowplow/snowplow/wiki/CLA).
+The @snowplowcla bot will guide you through the process.