Update docs_style_guide.md
Added link to the "Editing Documentation" page that explains the basics of PRs.
## Do's
- Use a GitHub Pull Request to submit documentation.
+ - See the [Editing Documentation](https://islandora.github.io/documentation/contributing/editing-docs/) documentation page for information on creating a Pull Request.
- Make it clear if the documentation is based on a particular configuration (such as Islandora Defaults) or if it applies to any deployment of Islandora.
- Submit documentation formatted in [Markdown](https://en.wikipedia.org/wiki/Markdown) format.
- Include a top-level heading for the whole page (using `#`)
