Update changelog

Author: phil-opp

Changelog.md

@@ -1,3 +1,9 @@
+## Breaking
+
+- Change the build system: Use a build script that expects a `KERNEL` environment variable instead of using a separate `builder` executable as before. See [#51](https://github.com/rust-osdev/bootloader/pull/51) and [#53](https://github.com/rust-osdev/bootloader/pull/53) for more information.
+  - This makes the bootloader incompatible with versions `0.6.*` and earlier of the `bootimage` tool.
+  - The bootloader also requires the `llvm-tools-preview` rustup component now.
+
 # 0.4.0
 
 ## Breaking