Installation instructions to build Python 2.7 (#1436)

Might be non-minimal

doc/installation_jetson_tx2_jetpack3.3.md

@@ -29,11 +29,30 @@ Notes:
 
 
 ## Installation
-Use the following script for installation of both caffe and OpenPose: 
+1. Use the following script for installation of both caffe and OpenPose: 
 ```
 bash ./scripts/ubuntu/install_caffe_and_openpose_JetsonTX2_JetPack3.3.sh
 ```
+2. If you do not want to build the Python libraries, the installation is finished. Otherwise, edit the BUILD_PYTHON flag on CMakeLists.txt:
 
+```option(BUILD_PYTHON "Build OpenPose python." ON)```
+
+and, in both places where this appears, set the flag to ON:
+```
+-DBUILD_python=ON
+-DBUILD_python_layer=ON
+````
+
+3. There are additional flags that need to be set: `PYTHON_EXECUTABLE=/usr/bin/python2.7` and `PYTHON_LIBRARY=/usr/lib/aarch64-linux-gnu/libpython2.7.so` for Python 2.7. Therefore, inside build, do:
+
+```cmake -DBUILD_PYTHON=ON -DPYTHON_EXECUTABLE=/usr/bin/python2.7 -DPYTHON_LIBRARY=/usr/lib/aarch64-linux-gnu/libpython2.7.so ..```
+
+4. Now run `make`.  You should see a file called "pyopenpose.so" if Python was set to 2.7, in
+/home/nvidia/openpose/build/python/openpose. Otherwise, it will be "pyopenpose.cpython-35m-aarch64-linux-gnu"
+
+5. Finally, run `sudo make install` inside build to copy the files to /usr/local/python and set PYTHONPATH accordingly on .bashrc:
+
+```export PYTHONPATH="${PYTHONPATH}:/usr/local/python```
 
 ## Usage
 It is for now recommended to use an external camera with the demo. To get to decent FPS you need to lower the net resolution: