Contributing to the Wiki documentation for Construct helps save the developer's time and gives you the opportunity to help the project! Many beginners need documentation so by writing even just a little you can be helping out a lot of users.
However, if you want to edit the Wiki, you need to sign up a SourceForge.net username, and post your username here. We have to approve individual editors to the Wiki - I've asked Sourceforge if it's possible to allow anonymous edits, but they don't seem to have added support for that yet.
You also need to visit the Wiki at least once with your newly signed up usersname. Otherwise, the wiki won't recognise you and we can't add you as an editor.
The wiki is the formal documentation for Construct, ie. a reference for all features. It'll also list tutorials and examples which are posted here. That makes it the #1 place to go for information on Construct!