Add actions status badge

Author: TG9541

README.md

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 # docker-sdcc
 [![Build and push Docker image](https://github.com/TG9541/docker-sdcc/actions/workflows/build-push.yml/badge.svg)](https://github.com/TG9541/docker-sdcc/actions/workflows/build-push.yml)
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 This repository contains all that's needed for building and testing TG9541/STM8EF:
 
 * SDCC 3.9.0 #11195 z180/r2k/r3ka/gbz80/tlcs90/ez80_z80/ds400/hc08/s08/stm8/pdk14/pdk15