Update changelog and Readme

Author: phil-opp

Diff for: Changelog.md

@@ -1,3 +1,5 @@
+- Make the physical memory offset configurable through a `BOOTLOADER_PHYSICAL_MEMORY_OFFSET` environment variable ([#58](https://github.com/rust-osdev/bootloader/pull/58)).
+
 # 0.6.0
 
 - **Breaking**: Don't set the `#[cfg(not(test))]` attribute for the entry point function in the `entry_point` macro

Diff for: README.md

@@ -64,6 +64,7 @@ The bootloader crate can be configured through some cargo features:
 - `vga_320x200`: This feature switches the VGA hardware to mode 0x13, a graphics mode with resolution 320x200 and 256 colors per pixel. The framebuffer is linear and lives at address `0xa0000`.
 - `recursive_page_table`: Maps the level 4 page table recursively and adds the [`recursive_page_table_address`](https://docs.rs/bootloader/0.4.0/bootloader/bootinfo/struct.BootInfo.html#structfield.recursive_page_table_addr) field to the passed `BootInfo`.
 - `map_physical_memory`: Maps the complete physical memory in the virtual address space and passes a [`physical_memory_offset`](https://docs.rs/bootloader/0.4.0/bootloader/bootinfo/struct.BootInfo.html#structfield.physical_memory_offset) field in the `BootInfo`.
+  - The virtual address where the physical memory should be mapped is configurable by setting the `BOOTLOADER_PHYSICAL_MEMORY_OFFSET` environment variable (supports decimal and hex numbers (prefixed with `0x`)).
 
 ## License