Closes #296 alt_text: add alt text to badge in README #648

Job	Run time
Get R version	1s
Links / Validate Links	7s
Check / (release)	6m 1s
Check / (devel)	44s
Check / (oldrel)	6m 20s
Documentation	0s
Validation	0s
Code Style / Code Style	2m 37s
Spelling / Spellcheck	3m 5s
Code Coverage / Test Coverage	3m 27s
Man Pages / Roxygen	3m 4s
Lint / Lint	2m 47s
Check Templates / Verify	3m 3s
Code Coverage / Generate badge for coverage	3s
	31m 19s

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