install ns3 ubuntu code example

Example: how to install ns3 in linux

## Installation
# Download ns3 from official site
./build.py --enable-examples --enable-test (in ns3-allinone-3.28 folder)
./waf --build-profile=debug --enable-examples --enable-tests configure(ns-3.28 Folder)
./waf 
./test.py -c core (optional)

## Running 
# (copy any .cc file from examples folder i.e. first.cc
# to scratch folder and run without .cc extension)
./waf --run scratch/first 

## python bindings
python ./waf configure

##Running in python
# (copy any .py file from examples folder i.e. first.py
# to scratch folder and run without .cc extension)
./waf --pyrun scratch/first .py