Add CONTRIBUTING.md

Author: michaelklishin

CONTRIBUTING.md

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+# Contributing
+
+This project uses a very minimalistic approach to contributions:
+
+ * Fork it
+ * Create a branch
+ * Make your changes
+ * Commit it with an explanation of [*why* this change is necessary](https://chris.beams.io/posts/git-commit/#why-not-how)
+ * Make sure all tests pass
+ * Submit a pull request
+ * Be patient
+
+## Prerequisites
+
+This project uses Gradle 6.x. A [compatible JDK version](https://docs.gradle.org/current/userguide/compatibility.html#java) is therefore required. 
+
+## Running Tests
+
+To start a local MongoDB node before running tests, use
+
+``` shell
+./gradlew startMongoDb
+```
+
+Then run the tests:
+
+``` shell
+./gradlew check
+```