## What changes were proposed in this pull request?

Updates links to the wiki to links to the new location of content on spark.apache.org.

## How was this patch tested?

Doc builds

Author: Sean Owen <sowen@cloudera.com>

Closes #15967 from srowen/SPARK-18073.1.

[SPARK-17840][DOCS] Add some pointers for wiki/CONTRIBUTING.md in README.md and some warnings in PULL_REQUEST_TEMPLATE

## What changes were proposed in this pull request?

Link to contributing wiki in PR template, README.md

## How was this patch tested?

Doc-only change, tested by Jekyll

Author: Sean Owen <sowen@cloudera.com>

Closes #15429 from srowen/SPARK-17840.

## What changes were proposed in this pull request?

(Please fill in changes proposed in this fix)

## How was the this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Author: Rahul Tanwani <tanwanirahul@gmail.com>

Closes #11343 from tanwanirahul/pull_request_template.