Create local-collaboration.md

I forget how to do this every single time 😅

docs/local-collaboration.md

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+# Local Collaboration
+
+## Setting up the local collaboration server
+
+### Setting up for the first time?
+
+Install [Postgres](https://postgresapp.com) and run it.
+
+Then, from the root of the repo, run `script/bootstrap`.
+
+### Have a db that is out of date? / Need to migrate?
+
+Try `cd crates/collab && cargo run -- migrate` from the root of the repo.
+
+## Testing collab locally
+
+1. Run `foreman start` from the root of the repo.
+1. In another terminal run `script/start-local-collaboration`.
+1. Two copies of Zed will open. Add yourself as a contact in the one that is not you.
+1. Start a collaboration session as normal with any open project.