docs: update CONTRIBUTING.m to document the NPM RUN REFORMAT step required in the PR checklist

It is possible to follow the CONTRIBUTING.md instructions, but still end up with an invalid PR due
to formatting issues. The PR checklist fixes this, but these instructions can be presented earlier
on in the process v

Author: skoloCFD

CONTRIBUTING.md

@@ -33,27 +33,34 @@ Please follow the coding style:
     $ git checkout -b new_feature
     ```
 
-4. Start hacking. Additional information on how to create class/test/example can be found
+6. Start hacking. Additional information on how to create class/test/example can be found
    [here](https://kitware.github.io/vtk-js/docs/) in the __Development__ section.
 
     ```sh
     $ edit file1 file2 file3
     $ git add file1 file2 file3
     ```
 
-5. Use Commitizen to create commits
+7. Verify you have correctly formatted code, and that all tests pass:
+
+    ```sh
+    $ npm run reformat
+    $ npm run test
+    ```
+
+8. Use Commitizen to create commits
 
     ```sh
     $ npm run commit
     ```
 
-6. Push commits in your feature branch to your fork in GitHub:
+9. Push commits in your feature branch to your fork in GitHub:
 
     ```sh
     $ git push origin new_feature
     ```
 
-7. Visit your fork in Github, browse to the "**Pull Requests**" link on the left, and use the 
+10. Visit your fork in Github, browse to the "**Pull Requests**" link on the left, and use the 
    "**New Pull Request**" button in the upper right to create a Pull Request.
 
     For more information see: 
@@ -62,7 +69,7 @@ Please follow the coding style:
 If committing changes to `vtk.js@next` or `vtk.js@next-major`, then set your base branch to be `next`
 or `next-major`, respectively. For more info see the section on release channels below.
 
-8. vtk.js uses GitHub for code review and Github Actions to validate proposed patches before they are merged.
+11. vtk.js uses GitHub for code review and Github Actions to validate proposed patches before they are merged.
 
 ## Release Channels