Exclude contrib folder from stats

[fntlnz]

This is useful to avoid cluttering the stats with the many additions that are done by directly committing generated files and downloaded dependencies into contrib.

Reference on linguist docs

• https://github.com/github/linguist/blob/0c7f82f88a2fb482db3a02812cbb4b8aab7308d1/docs/overrides.md#using-gitattributes