update docs

Author: leekillough

README.md

@@ -43,10 +43,12 @@ Building the Rev SST component from source using CMake (>= 3.19) can be performe
     git clone
     git config core.hooksPath .githooks
     cd rev/build
-    cmake -DRVCC=/path/to/riscv/compiler/exe ..
+    cmake -DRVCC=/path/to/riscv/c/compiler/exe -DRVCXX=/path/to/riscv/c++/compiler/exe ..
     make -j
     make install
 
+RVCC and RVCXX can be set as environment variables in lieu of passing them to CMake.
+
 Additional build options include:
     * -DBUILD_ASM_TESTING=ON
     * -DBUILD_DOCUMENTATION=ON  : enables Doxygen source code generation (use `make doc`)