Add documentation and pointers to online docs

for RenderMan Hydra plugin.

(Internal change: 1979133)

BUILDING.md

@@ -182,6 +182,20 @@ Embree 2.16.1. The additional dependencies that must be supplied when invoking c
 | --------------        | -----------------------------------         | -------   |
 | EMBREE_LOCATION       | The root path to an embree library install. | 2.16.1    |
 
+##### RenderMan Rendering
+
+This plugin uses Pixar's RenderMan as a rendering backend for Hydra and 
+usdview. Enable the plugin in the build by specifying the cmake flag 
+```PXR_BUILD_PRMAN_PLUGIN=TRUE``` when invoking cmake. This plugin is 
+compatible with RenderMan 22.5.  The additional dependencies that must be 
+supplied when invoking cmake are:
+
+| Dependency Name       | Description                                 | Version   |
+| --------------        | -----------------------------------         | -------   |
+| RENDERMAN_LOCATION    | The root path to an RenderMan install.      | 22.5      |
+
+More documentation is available [here](http://openusd.org/docs/RenderMan-USD-Imaging-Plugin.html).
+
 ## Third Party Plugins
 
 USD provides several plugins for integration with third-party software packages,

build_scripts/build_usd.py

@@ -1912,3 +1912,7 @@ def FormatBuildArguments(buildArgs):
 if context.buildHoudini:
     Print("See documentation at http://openusd.org/docs/Houdini-USD-Plugins.html "
           "for setting up the Houdini plugin.\n")
+
+if context.buildPrman:
+    Print("See documentation at http://openusd.org/docs/RenderMan-USD-Imaging-Plugin.html "
+          "for setting up the RenderMan plugin.\n")

third_party/renderman-22/README.md

@@ -2,3 +2,5 @@ RenderMan 22 Plugins
 ====================
 
 This section contains libraries and plugins for RenderMan 22. 
+
+See the [online documentation](http://openusd.org/docs/RenderMan-USD-Imaging-Plugin.html) for more details.