Add note on how to run tests in Contributing (#474)

Author: mhauru

tutorials/docs-01-contributing-guide/index.qmd

@@ -14,6 +14,21 @@ Look at the [issues](https://github.com/TuringLang/Turing.jl/issues) page for an
 
 For more information on how the Git workflow typically functions, please see the [GitHub's introduction](https://guides.github.com/introduction/flow/) or [Julia's contribution guide](https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md).
 
+### Tests
+
+Turing, like most software libraries, has a test suite. You can run the whole suite the usual Julia way with
+
+```julia
+Pkg.test("Turing")
+```
+
+The test suite subdivides into files in the `test` folder, and you can run only some of them using commands like
+
+```julia
+Pkg.test("Turing"; test_args=["optim", "hmc", "--skip", "ext"])
+```
+
+This one would run all files with "optim" or "hmc" in their path, such as `test/optimisation/Optimisation.jl`, but not files with "ext" in their path. Alternatively you can set these arguments as command line arguments when you run Julia, or otherwise set the global `ARGS` variable, and call `include("test/runtests.jl")`.
 
 ### Style Guide