Add CONTRIBUTING.md (close #1220)

CONTRIBUTING.md

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+# Contributor guide
+
+Iglu Central is maintained by the engineering team at Snowplow and improved by external contributors for which we are extremely grateful.
+
+## Submitting a PR
+
+1. Check if there is already an issue raised that describes why your change is needed. If there isn't an issue yet, please create one. If there is already an issue created, it's a good idea to comment and say that you're planning to work on it.
+2. Fork and clone `snowplow/iglu-central`.
+3. Create a branch with a relevant name e.g. `feature/sendgrid-events`.
+4. Add or patch schemas following the same layout as the repo's other schemas.
+5. Submit a Pull Request.
+
+## How to find out more information
+
+Find out more information on the [contributing page][docs-contributing] in our docs.
+
+## Sign our Contributor License Agreement
+
+We need all contributors to sign a CLA - you can find out why and how to sign it [here][cla].
+
+[docs-contributing]: https://docs.snowplow.io/docs/contributing/
+[cla]: https://docs.snowplow.io/docs/contributing/contributor-license-agreement/