READ ME: ld.elf_so changes

I've completed some extensive changes to ld.elf_so on MIPS:

* It can relocate itself (ELF_INTERP_NON_RELOCATABLE is no longer
  necessary).

* It actually does lazy binding now.  This is a significant
  performance win.

* Several errors WRT certain types of relocations are fixed.

Please test the new code, especially with large apps, and let me know
if you experience any problems.