Set gist visibility via Git push options (#215)

docs/usage/git-push-options.md

@@ -0,0 +1,13 @@
+# Push Options
+
+Opengist has support for a few [Git push options](https://git-scm.com/docs/git-push#Documentation/git-push.txt--oltoptiongt). 
+
+These options are passed to `git push` command and can be used to change the metadata of a gist.
+
+## Change visibility
+
+```shell
+git push -o visibility=public
+git push -o visibility=unlisted
+git push -o visibility=private
+```