Update gettingstarted.rst

Adds note on python 3 incompatibility
Added note about ansible_python_interpreter for people having python3 as default interpreter (mainly Arch users ?)
Should fix #2344

docsite/rst/gettingstarted.rst

@@ -39,6 +39,8 @@ also need:
    then allows you to use everything else.  (That's jumping ahead
    though.)
 
+   Ansible does not run with Python 3.
+
 Getting Ansible
 ```````````````
 
@@ -190,6 +192,9 @@ If you have python3 installed on Arch, you probably want to symlink python to py
 
     $ sudo ln -sf /usr/bin/python2 /usr/bin/python
 
+You can also use the ansible_python_interpreter variable for hosts that have python 
+pointing to python3 
+
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