Add a contributing file

Our open source policy dictates that every public repository should have
a contributing file. Add the default one here.

SEC-2232

CONTRIBUTING.md

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+# Contributing to logstash_logger_formatter
+
+We currently do not accept any contributions from external sources.
+
+The code is still open source and if you wish to add any changes, feel free to fork it and add changes to your fork.
+
+Issues for any security-related bugs found are still welcome, but we offer no guarantees on whether or when they will be acted upon.
+
+For any exceptional cases, please contact us at [email protected].