Document the directory layout

Add a new document: the test framework design document.
It only contains information about the source code layout for now.
It will be extended in the future.

Change-Id: I011ca17780e01d92363c69b06e72a7200b606a97

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+EL3 Firmware Test framework Design
+==================================
+
+Contents :
+
+1.  Code structure
+
+1.  Code Structure
+-------------------
+
+The code is organised into the following categories (present as directories at
+the top level):
+
+*   **Drivers.**
+*   **Framework.** Core features of the test framework.
+*   **Library code.** Comprise of functionality offered by the framework to the
+    tests.
+*   **Platform specific.** Choice of architecture-specific code depends upon
+    the platform.
+*   **Tests.**
+
+The tests are divided into the following categories (present as directories in
+the `tests/` directory):
+
+*   **Framework validation tests.** Tests that exercise the core features of
+    the framework. Verify that the test framework itself works properly.
+*   **Runtime services tests.** Tests that exercise the runtime services offered
+    by the EL3 Firmware to the Normal World software. This includes tests
+    against the Standard Service (to which PSCI belongs to) and the Trusted OS
+    service for now.
+*   **Tutorial tests.** Sample test code showing how to write tests in practice.
+    Serves as documentation of the tests API.
+
+All assembler files have the `.S` extension. The linker source file has the
+extension `.ld.S`. This is processed by GCC to create the linker script which
+has the extension `.ld`.