Add a stub Rust crate (#12595)

docs/development/contributing_guide.md

@@ -28,6 +28,9 @@ The source code of Synapse is hosted on GitHub. You will also need [a recent ver
 
 For some tests, you will need [a recent version of Docker](https://docs.docker.com/get-docker/).
 
+A recent version of the Rust compiler is needed to build the native modules. The
+easiest way of installing the latest version is to use [rustup](https://rustup.rs/).
+
 
 # 3. Get the source.
 
@@ -114,6 +117,11 @@ Some documentation also exists in [Synapse's GitHub
 Wiki](https://github.com/matrix-org/synapse/wiki), although this is primarily
 contributed to by community authors.
 
+When changes are made to any Rust code then you must call either `poetry install`
+or `maturin develop` (if installed) to rebuild the Rust code. Using [`maturin`](https://github.com/PyO3/maturin)
+is quicker than `poetry install`, so is recommended when making frequent
+changes to the Rust code.
+
 
 # 8. Test, test, test!
 <a name="test-test-test"></a>
@@ -195,7 +203,7 @@ The database file can then be inspected with:
 sqlite3 _trial_temp/test.db
 ```
 
-Note that the database file is cleared at the beginning of each test run. Thus it 
+Note that the database file is cleared at the beginning of each test run. Thus it
 will always only contain the data generated by the *last run test*. Though generally
 when debugging, one is only running a single test anyway.