conduwuit for FreeBSD

conduwuit at the moment does not provide FreeBSD builds. Building conduwuit on FreeBSD requires a specific environment variable to use the system prebuilt RocksDB library instead of rust-rocksdb / rust-librocksdb-sys which does not work and will cause a build error or coredump.

Use the following environment variable: ROCKSDB_LIB_DIR=/usr/local/lib

Such example commandline with it can be: ROCKSDB_LIB_DIR=/usr/local/lib cargo build --release