Add README to NeuralNetworks HAL

Adds README explaining where to find the NeuralNetworks sample driver.

Test: N/A
Change-Id: Ifbbbbb3801176da27fb03bbed0b0d739031a195b
diff --git a/neuralnetworks/README b/neuralnetworks/README
new file mode 100644
index 0000000..d8c8f5d
--- /dev/null
+++ b/neuralnetworks/README
@@ -0,0 +1,2 @@
+NeuralNetworks sample driver implementation is located at
+frameworks/ml/nn/driver/sample.