Added readme (WIP)

Author: cmaglie

README.md

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+# Arduino sketch (.ino) preprocessor
+
+This repository contains the source code of the Arduino sketch preoprocessor.
+The main purpose of this tool is to parse an Arduino sketch (usually made with the [Arduino IDE](https://github.com/arduino/Arduino)) and convert it into a valid C++ source code.
+
+This tool is based on the Clang/LLVM C/C++ parser and, among the sketch conversion process, provide some useful features used in the Arduino IDE like:
+
+* Diagnostics output in json format
+* Code autocomplete suggestions output in json format
+
+## Usage
+
+`./arduino-preprocessor sketch.ino.cpp -- [extra compiler options]`
+
+**TODO**: add command line options
+
+## The sketch (.ino) preprocessing
+
+**TODO**: Describe what happens exactly in the preprocessing
+
+* A sketch may be composed by many `.ino` files that are concatenated into a single `.ino.cpp` file
+* Libraries are detected and include path discovered
+* A pass of compiler with `-E` flag is made so all macros and include are resolved
+* The resulting file is preprocessed and missing prototypes are added
+
+## Credits
+
+This tool use code from the following projects:
+
+- [Clang/LLVM](http://llvm.org)
+- [JSON for Modern C++](https://github.com/nlohmann/json)
+