add little readme

complement/README.md

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+# Running Conduit on Complement
+
+This assumes that you're familiar with complement, if not, please readme
+[their readme](https://github.com/matrix-org/complement#running).
+
+Complement works with "base images", this directory (and Dockerfile) helps build the conduit complement-ready docker
+image.
+
+To build, `cd` to the base directory of the workspace, and run this:
+
+`docker build -t complement-conduit:dev -f complement/Dockerfile .`
+
+Then use `complement-conduit:dev` as a base image for running complement tests.