Add ATA PIO IRQ handler, documentation

Still WIP, doesn't look like IRQ handler works yet.
diff --git a/src/kernel/pic.c b/src/kernel/pic.c
index 5d33213..e4ed88c 100644
--- a/src/kernel/pic.c
+++ b/src/kernel/pic.c
@@ -51,6 +51,7 @@
 
 void pic_remap()
 {
+	// I don't remember what this does.
 	outb(0x20, 0x11);
 	outb(0xA0, 0x11);
 	outb(0x21, 0x20);