Write README

README.md

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+# LLVM with 6502 backend
+
+This is an [LLVM](http://llvm.org/) fork that tries to implements an [MOS
+6502](https://en.wikipedia.org/wiki/MOS_Technology_6502) backend.
+
+## Build
+
+Refer to the [Getting
+Started](http://llvm.org/docs/GettingStarted.html#getting-started-quickly-a-summary)
+guide from LLVM.
+
+A quick way to start (on Linux):
+
+  * Clone this repository
+  * Run inside directory:
+
+    mkdir build
+    cd build
+    cmake .. -DLLVM_TARGETS_TO_BUILD:STRING="X86;Mos6502"
+    make -j4
+
+If you want to build more targets, append them to the `LLVM_TARGETS_TO_BUILD`
+variable.  Adjust `-j` option of `make` in case you have more processor cores.
+
+## Install
+
+Run `make install` as usual.
+
+## Usage
+
+*TODO: Write how to convert from C/C++ with Clang to IR and 6502 assembly code.*
+
+## Tests
+
+To run regression tests for the MOS 6502 target, run from `build/`
+
+    make check-llvm-codegen-mos6502
+
+For more information on the LLVM test suite, refer to the [Testing
+Guide](http://llvm.org/docs/TestingGuide.html).

README.txt

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-Low Level Virtual Machine (LLVM)
-================================
-
-This directory and its subdirectories contain source code for LLVM,
-a toolkit for the construction of highly optimized compilers,
-optimizers, and runtime environments.
-
-LLVM is open source software. You may freely distribute it under the terms of
-the license agreement found in LICENSE.txt.
-
-Please see the documentation provided in docs/ for further
-assistance with LLVM, and in particular docs/GettingStarted.rst for getting
-started with LLVM and docs/README.txt for an overview of LLVM's
-documentation setup.
-
-If you're writing a package for LLVM, see docs/Packaging.rst for our
-suggestions.