Readme: Add reference to UnitfulParsableString.jl #559

Job	Run time
Documentation	4m 16s
Julia 1.0 - ubuntu-latest - x64	4m 7s
Julia 1.0 - macos-latest - x64	4m 32s
Julia 1.0 - windows-latest - x64	7m 49s
Julia 1.6 - ubuntu-latest - x64	3m 26s
Julia 1.6 - macos-latest - x64	5m 30s
Julia 1.6 - windows-latest - x64	13m 8s
Julia 1.10 - ubuntu-latest - x64	5m 34s
Julia 1.10 - macos-latest - x64	3m 24s
Julia 1.10 - windows-latest - x64	6m 17s
Julia 1 - ubuntu-latest - x64	4m 48s
Julia 1 - macos-latest - x64	4m 41s
Julia 1 - windows-latest - x64	3m 44s
Julia nightly - ubuntu-latest - x64	4m 28s
Julia nightly - macos-latest - x64	4m 46s
Julia nightly - windows-latest - x64	5m 15s
	1h 25m 45s

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