bootsrapping stages overview list

Author: mightyiam

Diff for: src/building/bootstrapping.md

@@ -18,6 +18,13 @@ rustc, then uses it to compile the new compiler.
 
 ## Stages of bootstrapping
 
+Overview:
+
+- Stage 0: the pre-compiled compiler
+- Stage 1: from current code, by an earlier compiler
+- Stage 2: the truly current compiler
+- Stage 3: the same-result test
+
 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.