README: add link to toolchain (#569)

Signed-off-by: Daniel Maslowski <[email protected]>

README.md

@@ -10,7 +10,8 @@ Building from repository
 -----------------------------
 
 We assume that the RISCV environment variable is set to the RISC-V tools
-install path, and that the riscv-gnu-toolchain package is installed.
+install path, and that the [riscv-gnu-toolchain](
+https://github.com/riscv-collab/riscv-gnu-toolchain) package is installed.
 
     $ git clone https://github.com/riscv/riscv-tests
     $ cd riscv-tests