Add warning about `LZ4FastCompressor` and `LZ4HCCompressor` #10

Job	Run time
Julia 1.6 - ubuntu-latest - x64 - pull_request	53s
Julia 1.6 - windows-latest - x64 - pull_request	5m 2s
Julia 1.6 - macos-latest - x64 - pull_request	1m 41s
Julia 1 - ubuntu-latest - x64 - pull_request	41s
Julia 1 - windows-latest - x64 - pull_request	1m 15s
Julia 1 - macos-latest - x64 - pull_request	1m 19s
	10m 51s

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