write intro

src/doc/unstable-book/src/the-unstable-book.md

@@ -1,2 +1,22 @@
 # The Unstable Book
 
+Welcome to the Unstable Book! This book consists of a number of chapters,
+each one organized by a "feature flag." That is, when using an unstable
+feature of Rust, you must use a flag, like this:
+
+```rust
+#![feature(box_syntax)]
+
+fn main() {
+    let five = box 5;
+}
+```
+
+The `box_syntax` feature [has a chapter][box] describing how to use it.
+
+[box]: box-syntax.html
+
+Because this documentation relates to unstable features, we make no guarantees
+that what is contained here is accurate or up to date. It's developed on a
+best-effort basis. Each page will have a link to its tracking issue with the
+latest developments; you might want to check those as well.