update resources

resources.md

@@ -11,6 +11,8 @@
 ## Boot sequence
 - <https://wiki.osdev.org/Boot_Sequence>
 - <https://wiki.osdev.org/Protected_Mode#Entering_Protected_Mode>
+- <https://wiki.osdev.org/Unreal_Mode>
+- <https://wiki.osdev.org/A20_Line>
 
 ## Memory
 - <https://wiki.osdev.org/Memory_Map_(x86)>
@@ -32,6 +34,7 @@
 - <https://en.wikipedia.org/wiki/Code_page_437>
 
 ## ELF
+- <http://www.skyfree.org/linux/references/ELF_Format.pdf>
 - <https://wiki.osdev.org/ELF>
 - <https://wiki.osdev.org/ELF_Tutorial>