chore: Add bootstrap tests to contrib doc (hyperium#1278)

CONTRIBUTING.md

@@ -206,6 +206,24 @@ example would explicitly use `Timeout::new`. For example:
 /// # }
 ```
 
+#### Generated code
+
+When making changes to `tonic-build` that affects the generated code you will
+need to ensure that each of the sub crates gets updated as well. Each of the sub
+crates like, for example `tonic-health`, generate their gRPC code via a
+`bootstrap.rs` test.
+
+The bootstrap tests work by generating the code and then checking git if there
+is any uncommitted generated code (there is a difference between the proto files
+and the committed generated code). At this point the test will fail telling you
+to commit the new code. When the new code is committed, running the test suite
+again will cause it to pass as the generated code doesn't create a diff for git
+and thus its up to date.
+
+```
+cargo test --all
+```
+
 ### Commits
 
 It is a recommended best practice to keep your changes as logically grouped as