Add README.md to cts/hostsidetests/multidevices

For easier discovery of go/cts-multi-device-module

Bug: 211047879
Test: TH
Change-Id: I10d29cef8c8d3b7ca1eef98d497ece691e4801c3
diff --git a/hostsidetests/multidevices/README.md b/hostsidetests/multidevices/README.md
new file mode 100644
index 0000000..2f42104
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+++ b/hostsidetests/multidevices/README.md
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+## CTS Multi-device Modules
+
+Refer to go/cts-multi-device-module for step-by-step instructions on creating
+CTS multi-device modules.
+