Update changelog for #61 and #62

Changelog.md

@@ -0,0 +1,2 @@
+- Add some basic documentation ([#62](https://github.com/rust-osdev/multiboot2-elf64/pull/62))
+- Add MemoryAreaType, to allow users to access memory area types in a type-safe way ([#61](https://github.com/rust-osdev/multiboot2-elf64/pull/61))