Add CONTRIBUTING.md, issue and pr templates

[monesidn]

Description

This PR introduces:

• a pretty simple CONTRIBUTING.md file;
• a small set of issue templates that we can refine later on;
• a PR template, used here to showcase it.

Fixes #170

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation

[coveralls]

coverage: 90.257%. remained the same
when pulling 8e16e46 on add-contributing-issue-and-pr-templates
into 78230c0 on master.