Merge pull request github#17426 from geoffw0/readme

Rust: Add a README.md

Author: redsun82

rust/README.md

@@ -0,0 +1,43 @@
+# Rust on CodeQL
+
+> [!WARNING]
+> Rust support for CodeQL is experimental. No support is offered. QL and database interfaces will change and break without notice or deprecation periods.
+
+## Development
+
+### Dependencies
+
+If you don't have the `semmle-code` repo you may need to install Bazel manually, e.g. from https://github.com/bazelbuild/bazelisk.
+
+### Building the Rust Extractor
+
+This approach uses a released `codeql` version and is simpler to use for QL development. From your `semmle-code` directory run:
+```bash
+bazel run @codeql//rust:rust-installer
+```
+You now need to create a [per-user CodeQL configuration file](https://docs.github.com/en/code-security/codeql-cli/using-the-advanced-functionality-of-the-codeql-cli/specifying-command-options-in-a-codeql-configuration-file#using-a-codeql-configuration-file) and specify the option:
+```
+--search-path PATH/TO/semmle-code/ql
+```
+(wherever the `codeql` checkout is on your system)
+
+You can now use the Rust extractor e.g. to run Rust tests from the command line or in VSCode.
+
+### Building the Rust Extractor (as a sembuild target)
+
+This approach allows you to build a Rust extractor with a CLI built from source. From your `semmle-code` directory run:
+```bash
+./build target/intree/codeql-rust
+```
+You can now invoke it directly, for example to run some tests:
+```bash
+./target/intree/codeql-rust/codeql test run ql/rust/ql/test/PATH/TO/TEST/
+```
+
+### Building a Database
+
+TODO
+
+### Code Generation
+
+TODO