Update README.md - Link to SDF Spec (#505)

Signed-off-by: Ethan Holter <[email protected]>
Co-authored-by: Ian Chen <[email protected]>

Author: ethanholter

README.md

@@ -51,6 +51,8 @@ See the [installation tutorial](https://gazebosim.org/api/sensors/9/installation
 
 Please refer to the [examples directory](https://github.com/gazebosim/gz-sensors/tree/main/examples).
 
+A list of sensors and SDF examples can be found in the [SDF specification](http://sdformat.org/spec?ver=1.12&elem=sensor)
+
 # Folder Structure
 
 Refer to the following table for information about important directories and files in this repository.