Coverage in CI and minor edit to README

[phil-davis]

1. minor edit to README
2. produce coverage in CI

[phil-davis]

https://github.com/sabre-io/vobject/pull/546/checks?check_run_id=3835938727

[2021-10-08T07:33:11.163Z] ['error'] There was an error running the uploader: No coverage files located, please try use `-f`, or change the project root with `-R`

I will have a look...

[codecov]

Codecov Report

Merging #546 (94e63e7) into master (d8a0a9a) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #546      +/-   ##
============================================
- Coverage     98.73%   98.68%   -0.05%     
- Complexity     1759     1778      +19     
============================================
  Files            66       66              
  Lines          4279     4425     +146     
============================================
+ Hits           4225     4367     +142     
- Misses           54       58       +4     

Impacted Files	Coverage Δ
lib/Cli.php	98.07% <ø> (+0.04%)	⬆️
lib/Property/Boolean.php	50.00% <0.00%> (-3.85%)	⬇️
lib/VCardConverter.php	98.40% <0.00%> (-0.53%)	⬇️
lib/Component.php	99.15% <0.00%> (-0.41%)	⬇️
lib/Parser/XML.php	97.50% <0.00%> (-0.04%)	⬇️
lib/Node.php	100.00% <0.00%> (ø)
lib/Document.php	100.00% <0.00%> (ø)
lib/StringUtil.php	100.00% <0.00%> (ø)
lib/FreeBusyData.php	98.59% <0.00%> (ø)
lib/Property/Uri.php	100.00% <0.00%> (ø)
... and 34 more

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