Document the Apache way development process

JuliaData/Arrow.jl has been transferred to apache/arrow-julia.
See also: #265 and https://issues.apache.org/jira/browse/INFRA-22663

We discussed the development process for apache/arrow-julia on dev@arrow.apache.org. See the VOTE thread for summary of it: https://lists.apache.org/thread/6q333y875v7mfyl2g988b01hqtgr52pt

apache/arrow-julia uses apache/arrow-rs like development process: https://docs.google.com/document/d/1TyrUP8_UWXqk97a8Hvb1d0UYWigch0HAephIjW7soSI/edit

Note: Contributors will be required to write issues for planned features and bug fixes so that we have visibility and opportunities for collaboration before a PR shows up. This is for the Apache way. See also http://theapacheway.com/ for the Apache way.

For example, apache/arrow-rs mentions it as comment in https://raw.githubusercontent.com/apache/arrow-rs/master/.github/pull_request_template.md . We need something like this to keep the Apache way development process.

Start by reviewing the development-process discussion and the apache/arrow-rs .github/pull_request_template.md reference, then inspect apache/arrow-julia's pull request template. Document the requirement to open issues for planned features and bug fixes, and verify that the resulting template guidance is clear to contributors.

Written by the indexing model from the issue text.