The technical requirements for those that wish to make changes to the chapters are fairly simple. You'll need either OpenOffice.org or LibreOffice (either of which can be downloaded for free) along with the Red Hat Liberation Fonts (these fonts are being used so that no one needs to worry about proprietary fonts).
The legal requirements are spelled out by the license that has been chosen for this book. This book is licensed under the GPL version 3.0 or later. If you don't feel like reading the legalese, the short of it is that you are free to download the book, share the book, and even modify the book, but if you choose to redistribute the book after you've modified it, you need to make your modifications available to others and they have the right as to whether or not they want to incorporate your modifications.
You are also required to submit only original or open source artwork along with any modifications you make. Please respect copyright laws.
While not a requirement, the Dmaths extension is highly recommended. It is useful for quickly inserting formulas into the text. You may need to translate Dmaths website as it is written in French.