Document proper usage of UDUNITS2_ROOT #886
Description
ngen's CMake project definition permits control over where UDUNITS2 is found via a user-provided UDUNITS2_ROOT value, but this isn't well documented. Also, only setting UDUNITS2_ROOT breaks builds for at least some environments, unless environment variables C_INCLUDE_PATH and CPLUS_INCLUDE_PATH are also set. This also isn't documented.
This relates to #725.