hello-world: Show how to run the tests in the instructions (#1618)

TomasHubelbauer · web-flow · commit a3d5356c461b · 2023-01-30T03:44:35.000Z
The track never introduces how to run tests to the student.

It is described in the HELP file but I believe that file is meant for
you to check when you get lost solving the exercise.

It should continue to contain the more detailed information on how to
run tests IMO but the basic command I believe should be a part of the
README.
diff --git a/exercises/practice/hello-world/.docs/instructions.append.md b/exercises/practice/hello-world/.docs/instructions.append.md
@@ -0,0 +1 @@
+In the Rust track, tests are run using the command `cargo test`.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+In the Rust track, tests are run using the command `cargo test`.