Explain lack of persistent storage for codespaces

Explain that any un-pushed changes in a codespace will be lost upon
its deletion.
Describe GitHub integration and pushing your work to GitHub as a good
remedy for potential data loss.

docs/getting-started/explanation/understanding-github-codespaces/index.md

@@ -28,6 +28,14 @@ to work on OpenSAFELY projects.
 
 GitHub provide users with a free monthly quota of Codespaces use.
 
+These cloud-hosted virtual machines have no persistent storage,
+which is to say any data on them will be lost when the machine is deleted
+if not saved elsewhere.
+Codespaces are primarily designed for writing code
+(such as an OpenSAFELY study)
+and are tightly integrated with GitHub,
+making it easy to commit and push your work to GitHub avoiding any data loss.
+
 ## Understanding GitHub Codespaces billing
 
 GitHub gives all users a free and decent-sized monthly quota for Codespaces.