Update README.md with a new section for the portable feature

README.md

@@ -61,3 +61,14 @@ for compatibility. If you need to modify column families concurrently, enable
 the crate feature `multi-threaded-cf`, which makes this binding's
 data structures use `RwLock` by default. Alternatively, you can directly create
 `DBWithThreadMode<MultiThreaded>` without enabling the crate feature.
+
+## Portable builds
+
+RocksDB's build, unlike Cargo, will default to setting `-march=native`, which
+generate code that fully takes advantage of build machine's CPU's features.
+This may create a binary that isn't compatible with other CPUs on the same
+architecture. For example, building on a machine with AVX512 support will create
+a binary that fails with `SIGILL` on machines without AVX512.
+
+Set the `portable` feature on this crate to pass `PORTABLE=1` to RocksDB's build,
+which will create a portable binary at the cost of some performance.