Update README.md

Adding PostgreSQL to README as an alternative requirement
Issue 3941: Code preview dark doesn't display correctly with long lines

Assigning the background color to the pre tag did not work correctly. Moved background color to the highlight class which fixes the problem.
fixes #3115 and adds support for gif bmp & tiff images

when showing image preview check via lowercased extension and adds support for gif bmp & tiff images
Merge pull request #3950 from bbodenmiller/patch-4

fixes #3115 and adds support for gif bmp & tiff images
Add background color to pre, too.
Lets cache branch_names, tag_names for repo
Explain yellow out-of-date button that sounds scary when read without context.
Move maintenance policy to help section.
Remove roadmap.
Merge branch 'dependencies-in-readme' of /home/git/repositories/gitlab/gitlabhq
Merge branch 'housekeeping-in-root-dir' of /home/git/repositories/gitlab/gitlabhq
Files controller handle redirect to remote storage. Added config block to carrierwave init
Merge pull request #3942 from bke-drewb/issue3941

Issue 3941: Code preview dark doesn't display correctly with long lines
Merge pull request #3940 from frlan/patch-3

Update README.md
better wording encouraging usage of latest stable release in GitLab Maintenance Policy
A little change on wording for new-user email

adds missing wording missed in https://github.com/gitlabhq/gitlabhq/commit/58d6176bc35fb45427df75b9762df6bb0d3b3a98 and https://github.com/gitlabhq/gitlabhq/pull/3902
Dont parse gfm-extractions as commit. Increase gfm rendering speed. Especially for a lot of code snippets
Merge pull request #3909 from bbodenmiller/patch-5

better wording encouraging usage of latest stable release in GitLab Maintenance Policy
Merge pull request #3961 from bbodenmiller/patch-1

A little change on wording for new-user email
improve regrex for match(sidekiq) robustness

e.g.: the sidekiq in my Debian box is v2.11.1