[BUILDING] make it clear for the user to set up path

BUILDING.md

@@ -366,16 +366,22 @@ Since CHERI support is available only for a few platforms, verifying CHERI-C
 programs that use header files from the C standard library will require a setup
 of a C standard library for one of these platforms.
 
+To obtain and install a CHERI sysroot, the
+[cheribuild](https://github.com/CTSRD-CHERI/cheribuild)
+tool is the recommended way:
+```
+cheribuild.py cheribsd-riscv64-purecap disk-image-riscv64-purecap -d
+```
+
+Once the build completed, you'll find `cheri` directory in your HOME directory.
+
 CHERI-enabled ESBMC defaults to the platform mips64-unknown-freebsd and
 expects the corresponding sysroot, the default of which can be configured by
 passing the CMake flags
 ```
 -DESBMC_CHERI_HYBRID_SYSROOT=<path> -DESBMC_CHERI_PURECAP_SYSROOT=<path>
 ```
 
-To obtain and install a CHERI sysroot, the
-[cheribuild](https://github.com/CTSRD-CHERI/cheribuild)
-tool is the recommended way:
-```
-cheribuild.py cheribsd-mips64-hybrid disk-image-mips64-hybrid
-```
+e.g. the 'path' should point to `$HOME/cheri/output/rootfs-riscv64-purecap`. As for the `rootfs-riscv64-purecap` part, you may want to use a diffrent directory if you used a different variant in the `cheribuild.py` command above.
+
+