Update readme file including ICBs shapefiles information

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Update readme file with a broad description of the initial Shiny app design. Also include details about the Shapefiles we will be using. Initially we will produce a dashboard based on English health geography. Seven NHS England (Regions) (NHSER) cover all of England, within which there are 42 Integrated Care Boards (ICB).

We will use publicly available statistics to create a shiny dashboard based on indicators for the 42 individual ICBs, hence we will be using ICBs shapefiles for our Leaflet maps.

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• This issue will cover adding to this project indicators and shapefiles required to build the Shiny dashboard. Displaying health and social indicators for Integrated Care Systems in England.

1-1. Indicators

• Health Index scores, Integrated care systems, England.
• Source: ONE URL: https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/healthandwellbeing/datasets/healthindexscoresintegratedcaresystemsengland

1-2. Shapefile

• Office for National Statistics > Open Geography Portal
• Open Geography Portal > Boundaries > Health Boundaries > Integrated Care Boards > 2023 Boundaries URL: https://geoportal.statistics.gov.uk/
• ICB (April 2023) EN BFC Shapefile https://geoportal.statistics.gov.uk/datasets/da81300d4b624a0b81376416c8b5d90e_0/explore

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Also modified readme file content, I have added new text describing data and maps sources and different packages we will use through the development phase.