Readme.md updates and cleanups
Readme.md updates and cleanups

Add more specific building notes.
Add MIPS directives.
Add built-in ARM and MIPS macros.
Add built-in functions.
Add links to references.
Clean up code samples.

sp1187/armips,Kingcom/armips,sp1187/armips,sp1187/armips,hrydgard/armips,Kingcom/armips,hrydgard/armips,Kingcom/armips,hrydgard/armips