Improve env vars instructions

[revolter]

No description provided.

[mollyIV]

Hey @revolter 👋 ,

I really like your PR and I think it will simplify a development process for issue-bot project 🙌

Recently we added a feature, where a bot sends a notification to Slack channel if pinned issue is closed: #63. I just noticed, that after this change, when we type bundle exec rake --tasks without having GITHUB_API_TOKEN ENV exported, we get the error 😿

Though your PR can be a good place to improve the docs for this case too.

What I would suggest is to move your new instructions about source .keys at the very top:

# Development

## Pre-requirements

1. create a `.keys` file containing:

export FASTLANE_ISSUE_BOT_SLUG="<github-username>/<repo-name>"
export GITHUB_API_TOKEN="<github-token>"
export ACTION_CHANNEL_SLACK_WEB_HOOK_URL="https://hooks.slack.com/services/<slack-hook-id>"

2. run `source .keys`

## Commands // didn't have better idea for this title, we can totally improve it
You can use rake to easily run a couple of tasks.
...

WDYT?

If you have no time working on that, I would be happy to take over and help 🙂