Rust: Add basic crate-level documentation

rust/flatbuffers/src/lib.rs

@@ -14,6 +14,20 @@
  * limitations under the License.
  */
 
+//! # FlatBuffers
+//!
+//! A library for memory-efficient serialization of data.
+//!
+//! This crate provides runtime support for the FlatBuffers format in the Rust programming language.
+//! To use this crate, first generate code with the `flatc` compiler, as described here: https://google.github.io/flatbuffers/
+//! Then, include that code into your project.
+//! Finally, add this crate to your `Cargo.toml`.
+//!
+//! At this time, Rust support is experimental, and APIs may change between minor versions.
+//!
+//! At this time, to generate Rust code, you will need the latest `master` version of `flatc`, available from here: https://github.com/google/flatbuffers
+//! (On OSX, you can install FlatBuffers from `HEAD` with the Homebrew package manager.)
+
 mod builder;
 mod endian_scalar;
 mod follow;