message: Update Readme.md for the current state and add some links to tracking issues
diff: ```bash
  $ git clone https://github.com/bjorn3/rustc_codegen_cranelift.git
  $ cd rustc_codegen_cranelift
  -$ rustup override set nightly
  -$ cargo install xargo
  -$ git submodule update --init
  +$ rustup override set nightly # This uses unstable api's which will never be stabilized
  +$ cargo install xargo # Used for building the sysroot
  +$ cargo install hyperfine # Used for benchmarking in build.sh
  $ cargo build
  ```
  ## Usage
  ```bash
  -$ rustc -Zcodegen-backend=$(pwd)/target/debug/librustc_codegen_cranelift.so my_crate.rs --crate-type lib -Og
  +$ rustc -Zcodegen-backend=$(pwd)/target/debug/librustc_codegen_cranelift.so my_crate.rs
  ```
  -## Building libcore
  +## Build sysroot and test
  ```bash
  -$ rustup component add rust-src
  -$ ./prepare_libcore.sh
  +$ rustup component add rust-src # Make sure the sysroot source is availablr
  +$ ./prepare_libcore.sh # Patch the sysroot source for some not yet supported things
  $ ./build.sh
  ```
  ## Not yet supported
  -* Checked binops
  -* Drop glue
  -
  -* Other call abi's
  -* Sub slice
  -
  -* Inline assembly
  -* Custom sections
  +* Good non-rust abi support ([non scalars are not yet supported for the "C" abi](https://github.com/bjorn3/rustc_codegen_cranelift/issues/10))
  +* Checked binops ([some missing instructions in cranelift](https://github.com/CraneStation/cranelift/issues/460))
  +* Inline assembly ([no cranelift support](https://github.com/CraneStation/cranelift/issues/444))
  +* Varargs ([no cranelift support](https://github.com/CraneStation/cranelift/issues/212))
  +* libstd (needs varargs and some other stuff) ([tracked here](https://github.com/bjorn3/rustc_codegen_cranelift/issues/146))
  +* u128 and i128 ([no cranelift support](https://github.com/CraneStation/cranelift/issues/354))
  +* SIMD (huge amount of work to get all intrinsics implemented, so may never be supported)
  