docs: add a few badges to README

[drmohundro]

Just adding a few https://swiftpackageindex.com/ badges.

Turns out the latest swiftlint also has some additional recommendations.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +8.64 🎉

Comparison is base (eacfa68) 65.41% compared to head (c3df866) 74.06%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #270      +/-   ##
==========================================
+ Coverage   65.41%   74.06%   +8.64%     
==========================================
  Files          36       36              
  Lines        1995     1997       +2     
==========================================
+ Hits         1305     1479     +174     
+ Misses        690      518     -172     

Impacted Files	Coverage Δ
...eserialization/XMLIndexer+XMLDeserialization.swift	75.00% <ø> (+15.25%)	⬆️
Source/Errors/XMLDeserializationError.swift	55.26% <ø> (+18.42%)	⬆️
Tests/SWXMLHashTests/LazyXMLParsingTests.swift	75.34% <ø> (+1.36%)	⬆️
...SWXMLHashTests/MixedTextWithXMLElementsTests.swift	60.00% <ø> (ø)
.../TypeConversionArrayOfNonPrimitiveTypesTests.swift	71.11% <ø> (+11.85%)	⬆️
...SWXMLHashTests/TypeConversionBasicTypesTests.swift	69.95% <ø> (+11.62%)	⬆️
...XMLHashTests/TypeConversionComplexTypesTests.swift	73.52% <ø> (+8.82%)	⬆️
...LHashTests/TypeConversionPrimitiveTypesTests.swift	67.39% <ø> (+13.04%)	⬆️
Tests/SWXMLHashTests/XMLParsingTests.swift	84.23% <ø> (+2.48%)	⬆️
...ts/SWXMLHashTests/LazyWhiteSpaceParsingTests.swift	76.19% <100.00%> (+1.19%)	⬆️
... and 1 more

... and 11 files with indirect coverage changes

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