install tensorflow for python 2 code example

Example 1: update tensorflow pip

######## for Python 2.7
pip install --upgrade pip
pip install --upgrade tensorflow

######## for Python 3.n
pip3 install --upgrade pip
pip3 install --upgrade tensorflow

Example 2: install tensorflow

pip install tensorflow

Example 3: installer tensorflow 2.0

# Requires the latest pip
pip install --upgrade pip

# Current stable release for CPU and GPU
pip install tensorflow

# Or try the preview build (unstable)
pip install tf-nightly