Document autogeneration

Signed-off-by: David Hotham <david.hotham@metaswitch.com>

Author: David Hotham

README.md

@@ -52,6 +52,17 @@ By default, `/usr/lib`, `/usr/local/lib64`, and `/usr/local/lib` are added to th
 
 A semicolon separated list of additional directories to add to the linker search path may be specified through the `CASSANDRA_SYS_LIB_PATH` environment variable.
 
+## Autogeneration
+
+The file `cassandra.rs` is autogenerated as follows:
+
+
+```
+$ bindgen --no-layout-tests --blacklist-type=max_align_t --output=src/cassandra.rs cassandra.h
+```
+
+The autogenerated code is formatted with `rustfmt`.
+
 
 ## History