Add link to 'Contributing' page in the top-level Arrow README

[asfimport]

It would be nice to link to the "Contributing to Apache Arrow" Confluence page(*EDIT) in the Sphinx docs directly from the main project README (in the already existing "Getting involved" section) because it's a bit hard to find right now.

"contributing" page: https://cwiki.apache.org/confluence/display/ARROW/Contributing+to+Apache+Arrow

main project README: https://github.com/apache/arrow/blob/master/README.md 

 

EDIT: Moving the "Contributing" wiki to a page in the actual Arrow Sphinx docs (location in repo) would also make it easier to find and modify. An additional task, ARROW-4427  was added to do this.

Reporter: Tanya Schlusser / @tanyaschlusser
Assignee: Wes McKinney / @wesm

Related issues:

• [Doc] Move Confluence Wiki pages to the Sphinx docs (is blocked by)

_{Note: This issue was originally created as ARROW-4425. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
I think we should actually put such documents in the Sphinx-generated docs. There's no reason to have separate wiki pages for that, IMHO.

[asfimport]

Tanya Schlusser / @tanyaschlusser:
Fair statement. Confluence is really hard for me to navigate. Updating and adding a sub-task.

[asfimport]

Wes McKinney / @wesm:
+1 for sphinx docs

[asfimport]

Wes McKinney / @wesm:
I'll take care of this along with a bunch of other documentation related tasks

[asfimport]

Wes McKinney / @wesm:
Resolved in d94a9fc