BLE: Add host test instructions.

features/FEATURE_BLE/tests/README.md

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+# mbed BLE host tests
+
+This folder contains tests for mbed BLE that can be run on an x86 host.
+
+## Getting started
+
+Run the following instructions to build the tests:
+
+```
+cd features/FEATURE_BLE/tests
+mkdir build
+cd build
+cmake ..
+make
+```
+
+The various tests applications are present at the root of the test folder;
+execute them to run the tests. As an examples gatt client related tests can be
+run with:
+
+```
+./gatt-client-tests
+```
+
+## Requirements
+
+These tests requires cmake on the host and a compliant C++14 compiler.
+
+## Resources
+
+These tests use extensively google test (gtest) and google mock. Please refer to
+the documentation of each products to get more insight:
+* gtest: https://github.com/google/googletest/tree/master/googletest/docs
+* gmock: https://github.com/google/googletest/tree/master/googlemock/docs
+