Include a simple README.txt in $HOME/.dart
Include a simple README.txt in $HOME/.dart

Fixes https://github.com/dart-lang/sdk/issues/42500

Change-Id: Iec9c6dd3d7c279ddd5eb9ba7fdb934ecd712ad69
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/158521
Reviewed-by: Michael Thomsen <5977a55c75bb4612004451a56b0b4c22d9d3311d@google.com>
Commit-Queue: Jaime Wren <cd1ab83d11dd618c6b8cd01592020d0bd5183fdc@google.com>

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