Add CONTRIBUTING.markdown (#30)

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

CONTRIBUTING.markdown

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+Contributing
+============
+
+Thanks for your interest in contributing to this project! We tried to make the
+contribution experience as simple as possible. This project can be built
+without requiring external dependencies other than C++20 compile and CMake as
+follows:
+
+```sh
+# On UNIX based systems
+make
+
+# On Windows (from a Visual Studio Developer Prompt)
+nmake
+```
+
+The default target will build the project and run the test suite.
+
+Grant of Rights
+---------------
+
+By contributing to this project, the Contributor irrevocably assigns,
+transfers, and conveys to Intelligence.AI all right, title, and interest in
+and to any contributions, including, but not limited to, all intellectual
+property rights.
+
+The contributor acknowledges and agrees that Intelligence.AI shall have the
+exclusive right to use, reproduce, modify, distribute, publicly display,
+publicly perform, sublicense, and otherwise exploit the contributions, whether
+in original or modified form, for any purpose and without any obligation to
+account to the contributor.
+
+The contributor grants Intelligence.AI the right to re-license the
+contributions, including any derivative works, under other licenses as
+Intelligence.AI deems appropriate.