write a bit about bootstrapping

src/overview.md

@@ -270,7 +270,23 @@ but there are already some promising performance improvements.
 
 ### Bootstrapping
 
-**TODO (or do we want such a section)?**
+`rustc` itself is written in Rust. So how do we compile the compiler? We use an
+older compiler to compile the newer compiler. This is called _bootstrapping_.
+
+Bootstrapping has a lot of interesting implications. For example, it means that one
+of the major users of Rust is Rust, so we are constantly testing our own
+software ("eating our own dogfood"). Also, it means building the compiler can
+take a long time because one must first build the compiler and then use that to
+build the new compiler (sometimes you can get away without the full 2-stage
+build, but for release artifacts you need the 2-stage build).
+
+Bootstrapping also has implications for when features are usable in the
+compiler itself. The build system uses the current beta compiler to build the
+stage-1 bootstrapping compiler. This means that the compiler source code can't
+use some features until they reach beta (because otherwise the beta compiler
+doesn't support them). On the other hand, for compiler intrinsics and internal
+features, we may be able to use them immediately because the stage-1
+bootstrapping compiler will support them.
 
 # Unresolved Questions