[docs] Update outdated ExtendingLLVM.rst

Summary:
The document is still incomplete in some degrees, but updated to reflect the
latest changes. Anyway we can detail it if any one think it is not enough. For
the sake of it, some useful examples are listed below:

Refer to r113618 "Add X86 MMX type to bitcode and Type" for how to add a new
type.

> One notable change from then is only one thing that ``lib/VMCore`` is renamed
to ``lib/IR``.

Refer to r194760 "Add addrspacecast instruction" for how to add a new
instruction.

Patch by Chilledheart (rwindz0@gmail.com).

Reviewed By: echristo

Differential Revision: http://reviews.llvm.org/D8897

llvm-svn: 234757