[HDRP] Added Vulkan install in system requirements (#3122)

* Added vulkan install in system requirements

* Reworded content

Co-authored-by: Lewis Jordan <lewis.jordan@hotmail.co.uk>

Author: alelievr

com.unity.render-pipelines.high-definition/Documentation~/System-Requirements.md

@@ -37,7 +37,7 @@ HRDP is only compatible with the following platforms:
 - MacOS (minimum version 10.13) using Metal graphics
 - Linux and Windows platforms with Vulkan
 
-**Note: HDRP only works on these platforms if the device used supports Compute Shaders. HDRP does not support OpenGL or OpenGL ES devices.**
+**Note: HDRP only works on these platforms if the device used supports Compute Shaders. HDRP does not support OpenGL or OpenGL ES devices. On Linux, Vulkan may not be installed by default. In that case you need to install it manually to run HDRP.**
 
 ### Ray tracing