API Documentation for notifications and README.md

[oxzi]

Add the bare minimum package documentation to ease external usage and finally drop a first README file.

---

README.md: Initial version

Add a first README.md file based on the layout used by Icinga DB.

For the moment, only the notifications package was mentioned as an
example, as others hopefully might start using this API after the
upcoming Notifications release.

References #62.

---

notifications: Initial Package Documentation

As the notifications API should be available for external users, a
minimum of API documentation would make a good first impression. This is
a start by adding package level documentation to the notifications
package and each subpackage.

An overview with references to subpackages was written for
notifications, while the user-facing packages got some pointers on how
to get started.

References #62.