Improvements in Readme

- Fix broken link to the issue tracker.
- Add contents section to make navigation easier throughout the page.
- Move the link to documentation contents near the top. Where it was
  before could be missed and documentation might seem inaccessible.

Change-Id: I502e4fc0fd312459cda351d30a8781c221625724
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>