chore: Add CONTRIBUTING.md with info on testing docs.rs docs

Author: jlizen

.gitignore

@@ -1,2 +1,3 @@
 /target
 Cargo.lock
+.vscode

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+## How to test docs.rs changes
+
+Set up your local docs.rs environment as per official README:  
+https://github.com/rust-lang/docs.rs?tab=readme-ov-file#getting-started
+
+Make sure you have:
+- Your .env contents exported
+- docker-compose for db and s3 running
+- The web server running via local (or pure docker-compose approach)
+- If on a remote machine, port 3000 (or whatever your webserver is listening on) forwarded
+
+Invoke the build command against your local path:
+
+```
+# you could also point to the `cratesfyi` binary from outside of the docker workspace,
+# though you'll still need the right ENVs exported
+cargo run -- build crate --local ../tokio-metrics
+```
+
+View the generated documentation for `tokio-metrics` in your browser