Add National Insurance documentation

[chelsea128]

Thanks for contributing to OpenFisca! Please remove this line, as well as, for each line below, the cases which are not relevant to your contribution :)

• Tax and benefit system evolution. | Technical improvement. | Crash fix. | Minor change.
• Impacted periods: all. | until YYYY-MM-DD. | from YYYY-MM-DD.
• Impacted areas: path/to/file/containing/impacted/variables
• Details:
  • New feature or new behaviour description
  • Cases for which an error was noticed

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These changes (remove lines which are not relevant to your contribution):

• Impact the PolicyEngine-UK public API (for instance renaming or removing a variable)
• Add a new feature (for instance adding a variable)
• Fix or improve an already existing calculation.
• Change non-functional parts of this repository (for instance editing the README)