Add README to devcontainers dir

Warn users not to modify and point to docs

.devcontainer/README.md

@@ -0,0 +1,9 @@
+# Devcontainer Configuration
+
+This folder contains configuration files required for Codespaces/devcontainers.
+Please do not modify its contents unless you know what you are doing and there
+is a specific change to the devcontainer configuration you need to make.
+
+If you believe you have a broken configuration or are unsure if you have the
+latest version of the configuration, please follow the 
+[instructions to add codespaces to your project](https://docs.opensafely.org/getting-started/how-to/add-github-codespaces-to-your-project/).