add a README explaining the purpose of the workspace/ folder

diff --git a/workspace/README.md b/workspace/README.md
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index 0000000..ff4e1ee
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+++ b/workspace/README.md
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+## Workspace
+
+This folder contains your Rust-101 workspace. I suggest you copy this
+folder somewhere else. Try `cargo build` in that new folder to check that compiling your workspace succeeds.
+(You can also execute it with `cargo run`, but you'll need to do some work before this will succeed.)
+See the [main tutorial](https://www.ralfj.de/projects/rust-101/main.html) for more details.
+
+Please do *not* edit this folder inside a `git clone` of Rust-101. The files in here are auto-generated.
+You should edit the files in `src/` in the repository root instead.