Create .gitattributes file to correctly classify the repository

[wbuchanan]

echo "*.do linguist-language=Python" >> .gitattributes

Repository is currently throwing off all the language stats for Stata. Simple fix and will correctly classify the repo.

[wbuchanan]

Project classified as Stata repo on GitHub, but repository does not appear to include any code related to the Stata language. Please correct the language classification for your project. It will help make your project more visible where it should be and will avoid coming up in searches for Stata projects for others.

[csrgxtu]

@wbuchanan thank you very much, i changed it according your comment.