Add section about knit tool

Author: bmarty

CONTRIBUTING.md

@@ -126,6 +126,23 @@ Note that you can run
 
 For ktlint to fix some detected errors for you (you still have to check and commit the fix of course)
 
+#### knit
+
+[knit](https://github.com/Kotlin/kotlinx-knit) is a tool which checks markdown files on the project. Also it generates/updates the table of content (toc) of the markdown files.
+
+So everytime the toc should be updated, just run
+<pre>
+./gradlew knit
+</pre>
+
+and commit the changes.
+
+The CI will check that markdown files are up to date by running
+
+<pre>
+./gradlew knitCheck
+</pre>
+
 #### lint
 
 <pre>