updating install.sh to create python 3 env and readme (CiscoDevNet#242)

* update readme

* update install.sh to create python3 env

* update readme with note about python envs

README.md

@@ -44,6 +44,11 @@ user-machine# brew install boost boost-python pkg-config cmake libssh
 ####Windows
 It is recommended to install a python distribution like [PythonXY](https://python-xy.github.io/) on your system before installing YDK-Py
 
+
+##Python Requirements:
+* Python 2.7 or 3.4
+
+
 ## Installation
 
 ```
@@ -175,6 +180,8 @@ YANG Development Kit Generator:
 
 ### Python Notes
 
+- If your environment has both python 2 and python 3 and uses python 2 by default, you may need to use 'python3' and 'pip3' instead of 'python' and 'pip' in the commands mentioned in this document.
+
 For Python entities and netconf session, CRUD service invoked on python class will:
 
 - Encode python data objects to netconf XML payload

install.sh

@@ -43,7 +43,7 @@ echo ""
 if [ -f "$MY_ENV/bin/activate" ]; then
     source $MY_ENV/bin/activate
 else
-    virtualenv -p python2.7 $MY_ENV
+    virtualenv -p python3 $MY_ENV
     source $MY_ENV/bin/activate
 fi