[GUIDE] How to create a new formula for HyperFormula engine

Description

For the contributors that would want to help finish the long "missing formulas" list.

Brainstorming:

• How a formula is build
• How they are grouped, which file should it be added to
• Where tests are located and how they are organized
• Checklist for completeness:
  • formula
  • tests
  • translations
  • meta data
• Where to find the spec (Open Document Format Formula spec)
• How to deal with incompatibilities with other spreadsheets if any
• CLA requirement
• Where to find the missing formulas list

Links

Might be a part of the readme https://github.com/handsontable/hyperformula/issues/8 "contributing" section.
Location for formulas, they are stored and grouped as plugins https://github.com/handsontable/hyperformula/tree/master/src/interpreter/plugin

This issue was transferred, the original author: @wojciechczerniak

Review the contributing section referenced in issue #8 and the formula plugins under src/interpreter/plugin. Map the existing formula, test, translation, metadata, specification, incompatibility, CLA, and missing-formula resources, then document where each belongs and how a contributor can verify a new formula is complete.