Document all crate features in the Readme

Author: phil-opp

Diff for: README.md

@@ -62,3 +62,5 @@ Where sdX is the device name of your USB stick. **Be careful** to choose the cor
 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`.