Add horizontal lines to rmarkdown table

I am trying to add horizontal lines to the following table and cannot figure out how to do it. Specifically, I want a line between rows 2 and 3 (Revision 2 and Revision 1)

| Revision | Author      | Date       | Description                                                                                                                                                                   |
|:----------------:|:---------------------------------:|:--------------------------:|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 2        | John Doe | 4/23/2019 | Updates to lots of official sounding stuff. 
| 
| 1        | John Doe | 4/25/2017  | Initial Release     

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