Improve contributing documentation

[David Jencks <da...@gmail.com>]

I’d like to verify my understanding of the jira and PR workflows and update the contributing.adoc page with some more details.

Here’s how I think camel works: 

- Camel operates with an informal review-then-commit policy for PRs.  If you are a committer, please wait for at least one approval before merging your PR.  If you are not a committer, usually the approver will merge your PR.

- Start by opening a JIRA issue for your work.  If several PRs will be involved due to scope, multiple branches, or multiple repositories, open a sub-task for each PR.

- Mention your jira issue or sub-task in the PR request, as in “resolves camel-123456789: ….”  Add a link to the PR in the jira issue or sub-task.

- After your PR is merged, make sure the PR is closed.

- After the PR is merged, mark the jira issue or subtask resolved.

Some projects immediately close jira issues, some mark them resolved and close them on release.  What does camel do?

Is there supposed to be a way for the jira issue or sub-task to automatically pick up PRs at GitHub?  I haven’t found a way to make this happen.  Is it supposed to work for apache gitbox PRs?

Is there anything else that I could discuss or add?

thanks!
David Jencks