[Docs] Add Shield.io badges

Author: danimart1991

README.md

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+![GitHub release (latest by date)](https://img.shields.io/github/v/release/danimart1991/pvpc-hourly-pricing-card)
+![GitHub last commit](https://img.shields.io/github/last-commit/danimart1991/pvpc-hourly-pricing-card)
+![License](https://img.shields.io/github/license/danimart1991/pvpc-hourly-pricing-card.svg)
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 # PVPC Hourly Pricing Card
 
 Home Assistant Lovelace custom card to use with [Spain electricity hourly pricing (PVPC) integration](https://www.home-assistant.io/integrations/pvpc_hourly_pricing/).