message: Add clone step to native build instrunction.
  Why:
  * Improve the documentation.
  This change addresses the need by:
  * Add clone repository step,
  * Change rpi_armv7 to rpi_armv6 .
diff: @@ -29,7 +29,7 @@ To cross compile TensorFlow Lite follow the steps:
  2. Clone TensorFlow repository:
  ```bash
  - git clone --depth 1 https://github.com/tensorflow/tensorflow.git tensorflow_src
  + git clone https://github.com/tensorflow/tensorflow.git tensorflow_src
  ```
  @@ -74,7 +74,14 @@ To natively compile TensorFlow Lite follow the steps:
  sudo apt-get install build-essential
  ```
  -2. Run following script at the root of the TensorFlow repository to download all the
  +2. Clone TensorFlow repository:
  +
  + ```bash
  + git clone https://github.com/tensorflow/tensorflow.git tensorflow_src
  +
  + ```
  +
  +3. Run following script at the root of the TensorFlow repository to download all the
  build dependencies:
  ```bash
  @@ -83,11 +90,11 @@ build dependencies:
  **Note:** You only need to do this once.
  -3. You should then be able to compile TensorFlow Lite with:
  +4. You should then be able to compile TensorFlow Lite with:
  ```bash
  ./tensorflow/lite/tools/make/build_rpi_lib.sh
  ```
  **Note:** This should compile a static library in:
  - `tensorflow/lite/tools/make/gen/lib/rpi_armv7/libtensorflow-lite.a`.
  + `tensorflow/lite/tools/make/gen/lib/rpi_armv6/libtensorflow-lite.a`.
  