Add submodule instructions; update submodule

- Update the problem-specifications submodule to the latest.
- While we're at it, update the README so others will not have to go
  hunting for the instructions to do the same!

README.md

@@ -4,6 +4,12 @@
 
 Exercism exercises in Perl 5
 
+## Getting started
+
+- Clone this repository
+- `git submodule update --init --recursive`.  This will pull in the
+  [Exercism problem specification] repository.
+
 ## Dependencies
 
 Each exercise has its own cpanfile (if necessary) for any dependencies it might have.
@@ -18,6 +24,14 @@ Each exercise should have an `exercise-data.yaml` file inside a directory named
 This is used by the `exercise-gen.pl` script in the `bin` directory to generate the module and test for an exercise.
 Documentation for the generator can be found in the `bin` directory.
 
+If the [Exercism problem specification] for the exercise has been updated,
+run
+
+    git submodule update --recursive --remote --checkout --force
+    git add problem-specifications
+
+to update the submodule to the latest.
+
 ## Testing the Tests
 
 The Exercism system allows us to have example implementations in the directory of each exercise.
@@ -50,3 +64,4 @@ The Perl Foundation does not support or endorse Exercism.
 
 [Carton]: https://metacpan.org/pod/Carton
 [contributing guide]: https://github.com/exercism/docs/blob/master/contributing-to-language-tracks/README.md
+[Exercism problem specification]: https://github.com/exercism/problem-specifications