OBS: don't build for EOLed Ubuntu releases

Stop building Trojita for Ubuntu 13.04 and 13.10 as they are EOL:
https://wiki.ubuntu.com/Releases

Change-Id: I2b075933f1ad22147f8b002c347e0136c4843c67
QtKeyChain library depends on Qt DBus library on non-apple unix systems

In QtKeyChain CMakeLists.txt is condition:

if (UNIX AND NOT APPLE)
  find_package(Qt4 COMPONENTS QtCore QtDBus REQUIRED)
endif()

So make sure that compilation of Trojita will not fail if is configured with
build option WITH_QTKEYCHAIN_PLUGIN. In that case add depends on WITH_DBUS.

For future there are planned KDE plugins which depends on DBus so make new
option WITH_DBUS instead adding another special checks for QtKeyChain.

Change-Id: Ifae261af3ee7e5fcef973e9635fa06a0f9acc00e
Merge "QtKeyChain library depends on Qt DBus library on non-apple unix systems"
Explain how to send patches to us

As per GitHub's James Dennes suggestion, let's use GitHub-common way [1] of
explaining how to contribute. Hopefully people will see this prior to creating
pull requests.

[1] https://github.com/blog/1184-contributing-guidelines

Change-Id: I0a2dfa596ff94953cdf0945bbcaa58af543dfffe