Move wiki documentation to the project

[jgribonvald]

We should provide the documentation into the project and move from the wiki https://wiki.jasig.org/display/PLT/Announcements+Portlet

[jgribonvald]

To works automaticaly with https://uportal-project.github.io/AnnouncementsPortlet we need a docs folder with a README.md file as entry point.