Add "Writing tests" section

Author: yegeunyang

src/doc/rustc-dev-guide/src/getting-started.md

@@ -137,6 +137,10 @@ pull request, continuing the work on the feature.
 
 [abandoned-prs]: https://github.com/rust-lang/rust/pulls?q=is%3Apr+label%3AS-inactive+is%3Aclosed
 
+### Writing tests
+
+Issues that have been resolved but do not have a regression test are marked with the `E-needs-test` label. Writing unit tests is a low-risk, lower-priority task that offers new contributors a great opportunity to familiarize themselves with the codebase.
+
 ### Contributing to std (standard library)
 
 See [std-dev-guide](https://std-dev-guide.rust-lang.org/).