Add rstudio subcommand docs

docs/opensafely-cli.md

@@ -195,6 +195,25 @@ opensafely exec IMAGE bash
 This can be useful if you want to explore the image manually.
 
 
+### `rstudio` - Running rstudio-server
+
+To launch the OpenSAFELY r image running in a local instance of rstudio-server please navigate to your research repository and run
+
+```bash
+opensafely rstudio
+```
+
+This will launch an rstudio-server session in a browser window, most likely at `http://localhost:8787/`. This provides the RStudio interface to the r image, therefore, the versions of R and all the packages are the same as they are in the r image.
+
+In your rstudio-server session you can work on your files as usual, including saving your edits.
+
+If you have an `.Rproj` file at the top level of your repository, `opensafely rstudio` will detect this and will open the rstudio-server session in RStudio's project mode. In this mode the Git pane is available, or you can make Git commits locally as you usually do.
+
+To end your rstudio-server session press ++ctrl+c++ in the Terminal window in which you ran the `opensafely run` command.
+
+An alternative method to open your project in rstudio-server is to open your research repository in [Codespaces](/getting-started/how-to/use-github-codespaces-in-your-project/#how-to-access-rstudio).
+
+
 ### `jupyter` - Running JupyterLab
 
 [Jupyter notebooks](https://jupyter.org/) are useful interactive