Update cargo feature description for "cache" in lib.rs

Author: bluss

openblas-src/src/lib.rs

@@ -6,7 +6,8 @@
 //!
 //! The following Cargo features are supported:
 //!
-//! * `cache` to build in `.cargo` instead of `target`,
+//! * `cache` to build in shared directory e.g. `$XDG_DATA_HOME/openblas_build/`
+//!   instead of `target` (see README),
 //! * `cblas` to build CBLAS (enabled by default),
 //! * `lapacke` to build LAPACKE (enabled by default),
 //! * `static` to link to OpenBLAS statically, and