Add docs for getting started writing your own Complement tests (#70)

Author: MadLittleMods

README.md

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+[![Complement Dev](https://img.shields.io/matrix/complement:matrix.org.svg?label=%23complement%3Amatrix.org&logo=matrix&server_fqdn=matrix.org)](https://matrix.to/#/#complement:matrix.org)
+
 ### Complement
 
 Complement is a black box integration testing framework for Matrix homeservers.
 
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+#### Getting started
+
+To get started developing, see https://github.com/matrix-org/complement/blob/master/ONBOARDING.md
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+If you're looking to run Complement against a local dev instance of Synapse, see [`matrix-org/synapse` -> `scripts-dev/complement.sh`](https://github.com/matrix-org/synapse/blob/develop/scripts-dev/complement.sh).
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+If you want to develop Complement tests while working on a local dev instance of Synapse, edit [`scripts-dev/complement.sh`](https://github.com/matrix-org/synapse/blob/develop/scripts-dev/complement.sh) to point to your local Complement checkout (`cd ../complement`) instead of downloading from GitHub.
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+
 #### Running
 
 You need to have Go and Docker installed. Then: