Add github-binarycache option to use built in cache, update readme #82

Job	Run time
test-vcpkg-action (ubuntu-20.04, x64-linux-release, false)	56s
test-vcpkg-action-latest-vcpkg-release (ubuntu-20.04, x64-linux-release, false)	53s
test-manifest (ubuntu-20.04, x64-linux-release, false)	39s
test-vcpkg-action (macos-11, x64-osx-release, false)	2m 12s
test-vcpkg-action-latest-vcpkg-release (macos-11, x64-osx-release, false)	45s
test-manifest (macos-11, x64-osx-release, false)	54s
test-vcpkg-action (windows-2019, x64-windows-release, false)	1m 48s
test-manifest (windows-2019, x64-windows-release, false)	1m 16s
test-vcpkg-action-latest-vcpkg-release (windows-2019, x64-windows-release, false)	1m 5s
test-vcpkg-action (ubuntu-20.04, x64-linux-release, true)	41s
test-vcpkg-action-latest-vcpkg-release (ubuntu-20.04, x64-linux-release, true)	43s
test-manifest (ubuntu-20.04, x64-linux-release, true)	42s
test-vcpkg-action (macos-11, x64-osx-release, true)	1m 3s
test-vcpkg-action-latest-vcpkg-release (macos-11, x64-osx-release, true)	46s
test-manifest (macos-11, x64-osx-release, true)	1m 8s
test-vcpkg-action (windows-2019, x64-windows-release, true)	1m 50s
test-vcpkg-action-latest-vcpkg-release (windows-2019, x64-windows-release, true)	1m 15s
test-manifest (windows-2019, x64-windows-release, true)	1m 26s
	20m 2s

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