[Website] Update arrow-site/README and any other places to point website contributors in right direction

[asfimport]

I noticed this is out of date

https://github.com/apache/arrow-site/blob/asf-site/README.md

We might want to ask ASF Infra to make the "master" branch the default when people land at apache/arrow-site on GitHub

Reporter: Wes McKinney / @wesm
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #12

_{Note: This issue was originally created as ARROW-6225. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
On apache/arrow-site#9, README.md gets deleted by the build script. There's no point of serving arrow.apache.org/README.md. Is that a satisfactory resolution?

Is ASF Infra required to change the default branch? Or is a committer or PMC authorized to go to the repository settings?

[asfimport]

Wes McKinney / @wesm:
Yeah Infra would have to change the default branch

[asfimport]

Neal Richardson / @nealrichardson:
I can make an Infra ticket–is that the way to go?

[asfimport]

Kouhei Sutou / @kou:
Could you create an issue to INFRA project to change the default branch?