Updated changelog for v0.12 (closes #78)

Changelog.md

@@ -1,3 +1,21 @@
+# 0.12.0
+
+- **breaking:** `load()` and `load_with_offset` now returns a result
+- added public constant `MULTIBOOT2_BOOTLOADER_MAGIC`
+- Rust edition 2018 (instead of 2015)
+- internal code improvements
+
+# 0.11.0
+
+- lib now contains public `TagType`-enum that contains
+  all possible mbi tags that are specified (taken from spec)
+- much improved debug-formatting of `BootInformation`
+- internal code improvements / formatting
+
+# 0.10.0
+- allow access to all memory regions (MemoryMap-Tag)
+- internal code improvements
+
 # 0.9.0
 
 - Add a `checksum_is_valid` method to the RSDP tags ([#64](https://github.com/rust-osdev/multiboot2-elf64/pull/64))