Add `Why Mill?` top-level section to readme #1827

Job	Run time
test-docs	5m 47s
build-linux / run	3m 33s
build-windows / run	6m 19s
compiler-bridge / run	1m 24s
lint-autofix / run	2m 39s
itest (11, ci/test-mill-dev.sh && ci/test-mill-release.sh && ./mill -i -k __.ivyDepsTree && ./mil... / run	3m 40s
itest (17, ci/test-mill-bootstrap.sh) / run	9m 18s
linux (11, '{scalajslib,scalanativelib,kotlinlib}.__.testCached') / run	16m 41s
linux (11, 'example.{depth,extending}.__.local.testCached') / run	7m 39s
linux (11, 'example.thirdparty[{mockito,acyclic,commons-io}].local.testCached') / run	4m 23s
linux (17, '{main,scalalib,testrunner,bsp,testkit}.__.testCached') / run	9m 31s
linux (17, 'example.javalib.__.local.testCached') / run	10m 29s
linux (17, 'example.kotlinlib.__.local.testCached') / run	18m 23s
linux (17, 'example.scalalib.__.local.testCached') / run	13m 34s
linux (17, 'example.thirdparty[{fansi,jimfs,netty,gatling}].local.testCached') / run	6m 27s
linux (17, 'integration.{failure,feature,ide}.__.local.testCached') / run	12m 31s
linux (17, 'integration.invalidation.__.fork.testCached') / run	14m 37s
linux (17, contrib.__.testCached) / run	4m 23s
linux (17, 'integration.invalidation.__.server.testCached') / run	9m 36s
windows (11, "{main,scalalib,bsp}.__.testCached") / run	14m 24s
windows (11, "example.scalalib.{basic,web}.__.fork.testCached") / run	12m 2s
windows (11, contrib.__.testCached) / run	15m 22s
windows (11, 'integration.invalidation[_].server.testCached') / run	13m 56s
windows (17, 'integration.{feature,failure}[_].fork.testCached') / run	14m 8s
	3h 50m 46s

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