Rust: Add a README.md

Author: geoffw0

rust/README.md

@@ -0,0 +1,42 @@
+# Rust on CodeQL
+
+## Development
+
+### Dependencies
+
+At present you need to be in the `rust-experiment` branch (or a branch on top of that) in both the `semmle-code` and `codeql` repos.
+
+TODO: are there any dependencies that aren't already CodeQL dependencies?  bazel and rustc?
+
+### 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 Users/YOURUSERNAME/semmle-code/ql
+```
+(or wherever the `ql` directory 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