git-coauthor: add page (#6878)

pages/common/git-coauthor.md

@@ -0,0 +1,9 @@
+# git coauthor
+
+> Add another author to the latest commit. Since this command rewrites the Git history, `--force` will be needed when pushing next time.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-coauthor>.
+
+- Insert an additional author to the last Git commit:
+
+`git coauthor {{name}} {{name@example.com}}`