The documentation appears in the following files:

API reference: API.md
Archiving email into Pony Mail: ARCHIVING.md
Building Pony Mail for Production: BUILDING.md
Contributing to Pony Mail: CONTRIBUTING.md
Imorting existing data into Pony Mail: IMPORTING.md
Installing:
    CentOS (and RHEL): INSTALL.centos.md
    Debian: INSTALL.debian.md
    Fedora: INSTALL.fedora.md
    Ubuntu: INSTALL.ubuntu.md
    Anywhere else: INSTALLING.md