Explicit dependencies in README

The README file explains how to build the system, but omits some important dependency information that has to be discovered as we build. It would be nice if there was a Dependencies section, explaining:

• libhandy is required
• modest is required, and the build system looks for myhtml even though the actual library is modest as per pkg-config (at least on my system ? Not on yours ?)
• libsecrets is a runtime dependency, as well as a backend for it (kwallet or gnome-keyring).