Adding a small document on the changes related to trusting proxy data
tweaked the PR check-list to make it more readable
added a shorter version of the todo list for minor changes
added more information about how to document BC breaks and deprecations
added more hints about the goals of the check-list
[#2128] Fixes per @WouterJ's suggestions
Merge pull request #2128 from symfony/trusting-proxies

Adding a small document on the changes related to trusting proxy data
Merge pull request #2137 from fabpot/contrib-updates

Tweak the PR check-list and add more information about how to contribute
updated security releases