Start a script to regenerate README.md from a template

So far this doesn't actually do any templating, just adds a note about
the README being autogenerated :)

src/bin/generate_readme.rs

@@ -0,0 +1,23 @@
+// This script reads README-template.md and generates the playground links
+// from the Rust source files in the various directories.
+
+// To add a new exercise, add it to the appropriate place in README-template.md
+// and then make sure to recompile this script (because the template gets
+// included at compile time and then run it to generate a new version of
+// README.md.
+
+use std::fs::File;
+use std::io::prelude::*;
+
+fn main() {
+    let template = include_str!("../../README-template.md");
+
+    let mut generated_readme = File::create("README.md").unwrap();
+    write!(generated_readme, "\
+<!-- This file was autogenerated by the script in src/bin/generate_readme.rs.
+     Please edit either the script or the template in README-template.md in
+     order to make changes here rather than committing the changes directly. -->
+
+").unwrap();
+    write!(generated_readme, "{}", template).unwrap();
+}