Add README explaining that this branch is empty.

README.txt

diff --git a/README.txt b/README.txt
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index 0000000..4f29060
--- /dev/null
+++ b/README.txt
@@ -0,0 +1,3 @@
+The master branch in this repository is empty
+
+Please check out one of the platform specific branches, like vim2-stable or imx8mevk-stable
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