* Reference redirectors and jenkins.io as much as possible

* Something went wrong, so this is a troubleshooting URL

* More specific redirect URLs

Next steps will be modifying the URL when jenkins.io is ready.

https://github.com/blog/1184-contributing-guidelines

Signed-off-by: Kamran Mackey <kamranm1200@gmail.com>

Signed-off-by: Kamran Mackey <kamranm1200@gmail.com>

Minor changes are always good.

Signed-off-by: Kamran Mackey <kamranm1200@gmail.com>

Remove extra lines under each title heading (forgot to add in previous commit).

Fix grammatical/spelling errors. Paste in copy of MIT License with recommended copyright format.

Removed an extra 'of' in line#5 to fix the sentence structure.
BEFORE: 'and a number of other "of" contributors
PROPOSED: and a number of other contributors

The goals is to maintain the information about the provided plugins near to the one showed on jenkins-ci.org home.

The number of provided plugins was outdated.