add style guide

source/handbook/marketing/developer-marketing/blog/style/index.md

+---
+layout: markdown_page
+title: "Developer Marketing - Blogging - Styleguide"
+---
+
+
+## Tone
+
+## Images
+
+## Referring to GitLab services
+
+- On-premises never on-premise
+
+
+## <a name="checklist"></a>Blog post publishing checklist
+
+Before you write, make sure you're on a new branch cloned from master.
+Check these before you publish:
+
+- First instance of GitLab should be linked to [GitLab](http://about.gitlab.com)
+- Check all links.
+- Check the date on the file.
+- Check the date in the post.
+- Check the image is crunched down. Use [tinypng](tinypng.com).
+- Check the blog appears good locally.