The PEAR Group occasionally publishes so called Administrative Documents, which describe the Group's decisions. You are expected to read those documents before starting serious contributions to PEAR, because they contain important information about the inner workings of PEAR. The developers mailing list will be informed, when new documents are added, so you do not need to visit the site frequently.
This manual contains the Developer Guide, which provides valueable information as well.
The Coding Standards describe the standards which PEAR code must adhere to.