Add a readme detailing the upstream llvm repository

README.md

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+# avr-llvm
+
+This repository has been merged into [upstream llvm](http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/AVR/).
+
+In order to compile stock LLVM will AVR support enabled, `-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=AVR` must be passed to CMake.
+
+## Compiling the up-to-date backend from upstream LLVM
+
+```bash
+# Clone the LLVM repository
+git clone https://github.com/llvm-mirror/llvm.git
+
+# optionally clone clang into the 'tools' folder for C/C++ compiler support
+git clone https://github.com/llvm-mirror/clang.git llvm/tools/clang
+
+# where we want to place object files/executables
+mkdir build && cd build
+
+# Run CMake to generate makefiles
+cmake ../llvm -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=AVR
+
+# Compile everything
+make
+```
+
+Once this is done, you will have a number executables in `build/bin`.
+