Create CONTRIBUTING.md

Author: LB--

CONTRIBUTING.md

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+Contributing Guidelines
+=======================
+
+This repository is for C++ code snippets, not complete programs. To add a snippet or set of related snippets, create a new folder (preferably all lowercase without spaces in the name) and add the snippets as C++ source or header files (please use .cpp and/or .hpp extensions). You must also include a README.md linking to e.g. ideone or coliru examples of working programs using the snippet(s), and a LICENSE file describing the license for the snippet(s).
+
+When your pull request is accepted, add a page to the wiki for the new snippet(s) describing how they work and how to use them.