Revert "remove distclean mention in the readme"

This reverts commit 38e144aeb7ac785ae68d1e7ee79ac4fa0b4412e2.
Revert "remove distclean (now that we've ported to mason and re-installs are fast)"

This reverts commit f8f38b100322aaf814d6a60a6d69776b446c1230.