Added .arcconfig

Summary:

• Phabricator URI: https://phabricator.wolfplex.be/
• Repo call sign: GFX
• History is mutable, so we can use arc amend in commit workflow
• No linter nor test

Mutability history reference:
https://secure.phabricator.com/book/phabricator/article/arcanist_new_project/#history-mutability-mercu

Test Plan: arc tasks to be sure I'm not on my default instance but on Wolfphab instance.

Reviewers: Kaliiixx

Reviewed By: Kaliiixx

Projects: GFX, Phabricator

Differential Revision: https://phabricator.wolfplex.be/D3