Mara Averick shows how to create a custom code snippet in RStudio:
The RStudio Support Code Snippets post is a great step-by-step for adding snippets of your own. The gist of it for a markdown snippet is as follows:
Open RStudio Preferences
Go to the Code section
Click the Edit Snippets button3
Select Markdown
Add your snippet and Save
Click through for a demo of how to embed tweets into your RMarkdown documentation.