Trusted Firmware v0.4 release documentation

Updates to readme.md and change-log.md to describe the features and changes in
the v0.4 release.

Change-Id: Ice0a7cf7abae349f552e662eac638a46acc5db3f
Document design of the Interrupt Mangement Framework

This patch adds documentation that describes the design of the Interrupt
management framework in the ARM Trusted Firmware. The porting-guide.md has also
been updated to describe the interface that should be implemented by each
platform to support this framework.

Change-Id: I3eda48e5c9456e6a9516956bee16a29e366633b7
Co-Authored-By: Soby Mathew <soby.mathew@arm.com>
Merge pull request #124 from 'danh-arm:sm/imf-documentation'
Merge pull request #122 from 'danh-arm:dh/v0.4-docs'