Add README for exerciser.

exerciser/README.md

@@ -0,0 +1,19 @@
+# exerciser
+
+This is a tool to generate templates for exercises from the Markdown source. Given a Markdown file
+with one or more sections like:
+
+````markdown
+<!-- File src/main.rs -->
+
+```rust,compile_fail
+{{#include example/src/main.rs:main}}
+
+fn some_more_code() {
+    // TODO: Write some Rust code here.
+}
+```
+````
+
+You can run it like `cargo run my/markdown/file.md exercise-templates/example`, and it will create a
+file `exercise-templates/example/src/main.rs` with the appropriate contents.