Update the readme with 32-bit info. (#29)

README.md

@@ -22,9 +22,17 @@ install path, and that the riscv-gnu-toolchain package is installed.
 
     $ mkdir build
     $ cd build
-    $ ../configure --prefix=$RISCV/riscv64-unknown-elf --host=riscv64-unknown-elf
+    $ ../configure --prefix=$RISCV --host=riscv64-unknown-elf
     $ make
     $ make install
 
 Alternatively, the GNU/Linux toolchain may be used to build this package,
 by setting `--host=riscv64-unknown-linux-gnu`.
+
+By default, 64-bit (RV64) versions of `pk` and `bbl` are built.  To
+built 32-bit (RV32) versions, supply a `--enable-32bit` flag to the
+configure command.
+
+The `install` step installs 64-bit build products into
+`$RISCV/riscv64-unknown-elf`, and 32-bit versions into
+`$RISCV/riscv32-unknown-elf`.