update ambiguous <OPTIONS> language in README to incl. mandatory USD path parameter

(nonspecific error when it's not included, referenced in #5)

Author: michaelybecker

README.md

@@ -141,6 +141,8 @@ where:
 | -DUSD_FILEFORMATS_FETCH_FASTFLOAT | Forces FetchContent for FastFLoat | ON | usdobj |
 | -DUSD_FILEFORMATS_ENABLE_ASM | Generate a ASM based material network on layerwrite | OFF |
 
+Note that `Dprx_ROOT` is *not* an optional parameter - a reference to the OpenUSD installation path **has** to be passed.
+
 ZLIB, Draco and OpenImageIO packages are hinted to search into the USD installation by default. Override this by setting their ROOT or their FETCH variables (no fetch for OIIO).
 
 The previous commands will place intermediate files into the folder `build` and install binaries into the folder `bin`.
@@ -257,4 +259,4 @@ To generate the documentation go to the project root folder and enter:
 ```
 doxygen
 ```
-The resulting documentation will be placed at the `docs` folder.
+The resulting documentation will be placed at the `docs` folder.