Add explanation how to test the book to contributing (#139)

CONTRIBUTING.md

@@ -1,11 +1,10 @@
 # Contributing
 
-## Discssion board
+## Discussion board
 
 If you have a question or an idea regarding certain content but you want to have feedback of fellow community members 
 and you think it may not be appropriate to file an issue open a discussion in our [discussion board](https://github.com/rust-unofficial/patterns/discussions).
 
-
 ## Writing a new article
 
 Before writing a new article please check our [issues](https://github.com/rust-unofficial/patterns/issues) and 
@@ -29,9 +28,7 @@ with the [Wayback Machine](https://web.archive.org/) and use the link to that sn
 
 Don't forget to add your new article to the `SUMMARY.md` to let it be rendered to the book.
 
-Please make `Draft Pull requests` early so we can follow your progress and can give early feedback (see the following section). 
-
-
+Please make `Draft Pull requests` early so we can follow your progress and can give early feedback (see the following section).
 
 ## Creating a Pull Request
 
@@ -42,6 +39,11 @@ Early reviews of the community are not meant as an offense but to give feedback.
 
 A good principle: "Work together, share ideas, teach others."
 
+### Test the book locally before submitting
+
+Before submitting the PR launch the commands `mdbook build` to make sure that the book builds and `mdbook test` to make sure that
+code examples are correct.
+
 ### Important Note
 
 Please **don't force push** your branch to keep commit history and make it easier of us to see changes between reviews.