Added CONTRIBUTING.md

Author: Vitaly Davydov

CONTRIBUTING.md

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+First off, thanks for taking the time to contribute! ❤️
+
+All types of contributions are encouraged and valued. See the Table of Contents for different ways to help and details about how this project handles them. Please make sure to read the relevant section before making your contribution. It will make it a lot easier for us maintainers and smooth out the experience for all involved. The community looks forward to your contributions.
+
+# Table of Contents
+- [Project Scope](#project-scope)
+- [Development Plans](#development-plans)
+
+# Project Scope
+- Types.
+- Operations on types (creating, searching, sorting, set operations and others).
+- Indexing structures and algorithms.
+
+# Development Plans
+- Move all source files into src folder.
+- Implement optional compilation with HEALPIX and SMOC.
+- Use pgindent to format sources.
+- Check compilation with newest postgrespro versions.
+- Add more tests including performance tests.
+- Add more documentation.