Remove reference to "develop" branch
We just work off of master now.
You can work on the ATC without a full Concourse deployment. This is useful for testing changes to the web UI. The only limitation is that any builds you run will fail with the `no workers` error. To test your local changes with a full deployment, follow the instructions in the [concourse/concourse CONTRIBUTING.md](https://github.com/concourse/concourse/blob/develop/CONTRIBUTING.md) instead.
## Checkout the code
-You need to checkout the `concourse/concourse` repo. `atc` will be picked up as a submodule. The `develop` branch is for active work:
+You need to checkout the `concourse/concourse` repo. `atc` will be picked up as a submodule.
```
-$ git clone https://github.com/concourse/concourse
-git checkout develop
+git clone https://github.com/concourse/concourse
+cd concourse
git submodule update --init --recursive
```
