Add bootstrapping diagram

src/building/bootstrapping.md

@@ -18,7 +18,32 @@ rustc, then uses it to compile the new compiler.
 
 ## Stages of bootstrapping
 
-Compiling `rustc` is done in stages.
+Compiling `rustc` is done in stages. Here's a diagram, adapted from Joshua Nelson's
+[talk on bootstrapping][rustconf22-talk] at RustConf 2022, with detailed explanations below.
+
+The `A`, `B`, `C`, and `D` show the ordering of the stages of bootstrapping.
+<span style="background-color: lightblue">Blue</span> nodes are downloaded,
+<span style="background-color: yellow">yellow</span> nodes are built with the stage0 compiler, and
+<span style="background-color: lightgreen">green</span> nodes are built with the stage1 compiler.
+
+[rustconf22-talk]: https://rustconf.com/schedule#bootstrapping-the-once-and-future-compiler
+
+```mermaid
+graph TD
+    s0c["stage0 compiler (1.63)"]:::downloaded -->|A| s0l("stage0 std (1.64)"):::with-s0c;
+    s0c & s0l --- stepb[ ]:::empty;
+    stepb -->|B| s0ca["stage0 compiler artifacts (1.64)"]:::with-s0c;
+    s0ca -->|copy| s1c["stage1 compiler (1.64)"]:::with-s0c;
+    s1c -->|C| s1l("stage1 std (1.64)"):::with-s1c;
+    s1c & s1l --- stepd[ ]:::empty;
+    stepd -->|D| s1ca["stage1 compiler artifacts (1.64)"]:::with-s1c;
+    s1ca -->|copy| s2c["stage2 compiler"]:::with-s1c;
+
+    classDef empty width:0px,height:0px;
+    classDef downloaded fill: lightblue;
+    classDef with-s0c fill: yellow;
+    classDef with-s1c fill: lightgreen;
+```
 
 ### Stage 0