sug: document what has to be done to use a custom build of NukeUsdPlugins

[davvid]

We use a custom build of USD.

In Nuke13 I had to manually remove various plugins from the Nuke installation to prevent them from loading. I then built NukeUsdPlugins and added it to the NUKE_PATH and things worked as expected.

Here are all of the files I had to remove from the nuke installation in order to get things working with a custom USD on Linux (Nuke 13.0, but it should be be similar in later versions):

libarch.so        libhgiInterop.so  libtrace.so         libusdShade.so
libar.so          libhgi.so         libusdAppUtils.so   libusdSkelImaging.so
libcameraUtil.so  libhio.so         libUsdConverter.so  libusdSkel.so
libgarch.so       libjs.so          libusdGeom.so       libusd.so
libgf.so          libkind.so        libusdHydra.so      libusdUI.so
libglf.so         libndr.so         libusdImagingGL.so  libusdUtils.so
libHdNuke.so      libpcp.so         libusdImaging.so    libusdVolImaging.so
libhd.so          libplug.so        libusdLux.so        libusdVol.so
libhdSt.so        libpxOsd.so       libusdMedia.so      libvt.so
libhdx.so         libsdf.so         libusdRender.so     libwork.so
libhf.so          libsdr.so         libusdRiImaging.so  usd
libhgiGL.so       libtf.so          libusdRi.so

(usd is a directory)

As expected, these are all of the USD libraries and a few of the NukeUsdPlugins libraries.

I had to manually remove these from the Nuke installation in order to get things working.

IMO this is expected since I'm basically recompiling those parts, but this might be a bit advanced for someone that isn't familiar with USD or nuke plugins.

Assuming that this is an expected / supported use case (please let me know if there's something I'm missing), would you accept a pull request that adds documentation to the README showing how to modify a Nuke installation so that an externally-provided USD and NukeUsdPlugins can be used?

Thanks for making these plugins available.