git-setup: add page (#6057)

pages/common/git-setup.md

@@ -0,0 +1,13 @@
+# git setup
+
+> Create a Git repository in a specific directory and commit all files.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-setup>.
+
+- Create a Git repository in the current directory and commit all files:
+
+`git setup`
+
+- Create a Git repository in a specific directory and commit all files:
+
+`git setup {{path/to/directory}}`