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ARROW-6489: [Developer][Documentation] Fix merge script and readme

The following things are fixed:

- Specify `--ff` option to `git merge` command to prevent the effect of git's `merge.ff` value
- Write about `APACHE_JIRA_USERNAME` and `APACHE_JIRA_PASSWORD` environment variables in `dev/README.md`
- Describe the users need to install `requests` and `jira` libraries before running `dev/merge_arrow_pr.py`

Closes #5325 from mrkn/ARROW-6489 and squashes the following commits:

89d49d11f <Kenta Murata> Describe how to specify JIRA account information
604c659e4 <Kenta Murata> Describe the requirements of the merge script
5e5680e99 <Kenta Murata> Use --ff option to prevent the effect of git's merge.ff configuration

Authored-by: Kenta Murata <mrkn@mrkn.jp>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>