Add slang-llvm and slang-glslang readmes (#4646)

* Add slang-llvm and slang-glslang readmes

Just extraced and updated from the original repos

* Remove redundant file

1 files changed, 30 insertions, 0 deletions

diff --git a/source/slang-llvm/README.md b/source/slang-llvm/README.md
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+Slang LLVM/Clang Library
+========================
+
+The purpose of this project is to use the [LLVM/Clang infrastructure](https://github.com/shader-slang/llvm-project/) to provide features for the [Slang language compiler](https://github.com/shader-slang/slang/). 
+
+These features may include
+
+* Use as a replacement for a file based downstream C++ compiler for CPU targets
+* Allow the 'host-callable' to generate in memory executable code directly
+* Allow parsing of C/C++ code 
+* Compile Slang code to bitcode 
+* JIT execution of bitcode
+
+Currently only executing code via 'host-callable' mechanism is supported.
+
+How to use
+==========
+
+If the `slang-llvm` shared library/dll is available to Slang, Slang will automatically use LLVM JIT for `host-callable` compilations.
+
+Limitiations
+============
+ 
+* Only supports `host-callable`
+
+Building LLVM/Clang
+===================
+
+This repo's `external/build-llvm.sh` script builds llvm with the correct
+options to be used by slang, please refer to that.