Added README.md

Author: mikeller

README.md

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+# Blackbox Explorer Nightly Builds
+
+Download installable versions of the latest state of Blackbox Explorer for Windows / MacOS / Linux from here.
+
+See https://github.com/betaflight/blackbox-log-viewer for instructions.
+
+**Caution: These versions are unstable, and can result in misconfigurations / data loss. Use only for testing.**