Define guidelines

[ctmbl]

As introduced with #58 (comment) it's time to deal with Guidelines

Ideas in no particular order:

• CONTRIBUTING.md
• how to write a good title, a good summary
• naming guidelines for article files (upper case, dash/underscore, length, keyword) to produce consistent and informative URLs
• specials file name and title format for write up
• non-exhaustive set of tags to control tag proliferation
• tag rules (upper case, length,...)
• non-english articles: tags, title (see Add language in title when not in English #63 )
• enforce rules or check them through workflow

this must be discuss and order before starting a pull request