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.
Add background color to pre, too.
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
deep sub-directory support for group/namespace

so group path could be set freely such as:
    a -> a/b/c,
    a/b/c -> a

useful to host some complicated repositories tree layout,
e.g.: Google Android repositories tree.
improve regrex for match(sidekiq) robustness
misc refined and random fixed
all in one script to install gitlab/ci on debian/ubuntu