Update README

README.md

@@ -1,20 +1,30 @@
-RISC-V Proxy Kernel
-=====================
+RISC-V Proxy Kernel and Boot Loader
+=====================================
 
 About
 ---------
 
-The RISC-V proxy kernel is a thin layer that services system calls generated
-by code built and linked with the RISC-V newlib port.
+The RISC-V Proxy Kernel, `pk`, is a lightweight application execution
+environment that can host statically-linked RISC-V ELF binaries.  It is
+designed to support tethered RISC-V implementations with limited I/O
+capability and and thus handles I/O-related system calls by proxying them to
+a host computer.
+
+This package also contains the Berkeley Boot Loader, `bbl`, which is a
+supervisor execution environment for tethered RISC-V systems.  It is
+designed to host the RISC-V Linux port.
 
 Build Steps
 ---------------
 
 We assume that the RISCV environment variable is set to the RISC-V tools
-install path, and that the riscv-gcc package is installed.
+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
     $ make
     $ make install
+
+Alternatively, the GNU/Linux toolchain may be used to build this package,
+by setting `--host=riscv64-unknown-linux-gnu`.